Fix unexpected output from 'printf %T' with certain formats (#65)

This commit changes the behavior of four date formats accepted
by 'printf %()T' because the old behavior is not compatible with
modern implementations of date(1):
- %k and %l now return a blank-padded hour, the former based on a
  24-hour clock and the latter a 12-hour clock (these are common
  extensions present on Linux and *BSD).
- %f now returns a date with the format '%Y.%m.%d-%H:%M:%S'
  (BusyBox extension).
- %q now returns the quarter of the current year (GNU extension).

src/cmd/ksh93/data/builtins.c:
- Copy the date format documentation from date in libcmd to
  the printf man page (for documenting 'printf %T').

src/cmd/ksh93/tests/builtins.sh:
- Add four regression tests for the changed date formats.

src/cmd/ksh93/sh.1:
- Remove inaccurate information about the date formats accepted by
  printf %T'. The KornShell uses a custom version of strftime(3)
  that isn't guaranteed to accepts the same formats as the native
  strftime function.

src/lib/libast/tm/tmxfmt.c:
- Change the behavior of %f, %k, %l and %q to the common behavior.
  %k and %l are implemented as aliases to %_H and %_I to avoid
  duplicating code.

src/lib/libcmd/date.c:
- Update the documentation for the AST date command since it is
  also affected by the changes to 'printf %T'.

Fixes #62

src/cmd/ksh93/data/builtins.c

@@ -1221,8 +1221,74 @@ USAGE_LICENSE
 		"convert it to an extended regular expression.]"
 	"[+%T?Treat \astring\a as a date/time string and format it.  The "
 		"\bT\b can be preceded by \b(\b\adformat\a\b)\b, where "
-		"\adformat\a is a date format as defined by the \bdate\b "
-		"command.]"
+		"\adformat\a is a date format. The accepted date formats are "
+		"as follows:]{"
+			"[+%?% character]"
+			"[+a?abbreviated weekday name]"
+			"[+A?full weekday name]"
+			"[+b?abbreviated month name]"
+			"[+B?full month name]"
+			"[+c?\bctime\b(3) style date without the trailing newline]"
+			"[+C?2-digit century]"
+			"[+d?day of month number]"
+			"[+D?date as \amm/dd/yy\a]"
+			"[+e?blank padded day of month number]"
+			"[+f?print a date with the format '\%Y.\%m.\%d-\%H:\%M:\%S']"
+			"[+F?%ISO 8601:2000 standard date format; equivalent to Y-%m-%d]"
+			"[+g?\bls\b(1) \b-l\b recent date with \ahh:mm\a]"
+			"[+G?\bls\b(1) \b-l\b distant date with \ayyyy\a]"
+			"[+h?abbreviated month name]"
+			"[+H?24-hour clock hour, zero-padded]"
+			"[+i?international \bdate\b(1) date with time zone type name]"
+			"[+I?12-hour clock hour, zero-padded]"
+			"[+j?1-offset Julian date]"
+			"[+J?0-offset Julian date]"
+			"[+k?24-hour clock hour, blank-padded]"
+			"[+K?all numeric date; equivalent to \b%Y-%m-%d+%H:%M:%S\b; \b%_[EO]]K\b for space separator, %OK adds \b.%N\b, \b%EK\b adds \b%.N%z\b, \b%_EK\b adds \b.%N %z\b]"
+			"[+l?12-hour clock hour, blank-padded]"
+			"[+L?locale default date format]"
+			"[+m?month number]"
+			"[+M?minutes]"
+			"[+n?newline character]"
+			"[+N?nanoseconds 000000000-999999999]"
+			"[+p?meridian (e.g., \bAM\b or \bPM\b)]"
+			"[+q?quarter of the year]"
+			"[+Q?\a<del>recent<del>distant<del>\a: \a<del>\a is a unique "
+				"delimter character; \arecent\a format for recent "
+				"dates, \adistant\a format otherwise]"
+			"[+r?12-hour time as \ahh:mm:ss meridian\a]"
+			"[+R?24-hour time as \ahh:mm\a]"
+			"[+s?number of seconds since the epoch; \a.prec\a preceding "
+				"\bs\b appends \aprec\a nanosecond digits, \b9\b if "
+				"\aprec\a is omitted]"
+			"[+S?seconds 00-60]"
+			"[+t?tab character]"
+			"[+T?24-hour time as \ahh:mm:ss\a]"
+			"[+u?weekday number 1(Monday)-7]"
+			"[+U?week number with Sunday as the first day]"
+			"[+V?ISO week number (i18n is \afun\a)]"
+			"[+w?weekday number 0(Sunday)-6]"
+			"[+W?week number with Monday as the first day]"
+			"[+x?locale date style that includes month, day and year]"
+			"[+X?locale time style that includes hours and minutes]"
+			"[+y?2-digit year (you'll be sorry)]"
+			"[+Y?4-digit year]"
+			"[+z?time zone \aSHHMM\a west of GMT offset where S is "
+				"\b+\b or \b-\b, use pad _ for \aSHH:MM\a]"
+			"[+Z?time zone name]"
+			"[+=[=]][-+]]flag?set (default or +) or clear (-) \aflag\a "
+				"for the remainder of \aformat\a, or for the remainder "
+				"of the process if \b==\b is specified. \aflag\a may be:]{"
+				"[+l?enable leap second adjustments]"
+				"[+n?convert \b%S\b as \b%S.%N\b]"
+				"[+u?UTC time zone]"
+			"}"
+			"[+#?equivalent to %s]"
+			"[+??alternate?use \aalternate\a format if a default format "
+				"override has not been specified, e.g., \bls\b(1) uses "
+				"\"%?%l\"; export TM_OPTIONS=\"format='\aoverride\a'\" "
+				"to override the default]"
+		"}"
 	"[+%Z?Output a byte whose value is \b0\b.]"
 	"\fextra\f"
 "}"

src/cmd/ksh93/include/version.h

@@ -17,4 +17,4 @@
 *                  David Korn <dgk@research.att.com>                   *
 *                                                                      *
 ***********************************************************************/
-#define SH_RELEASE	"93u+m 2020-07-06"
+#define SH_RELEASE	"93u+m 2020-07-07"

src/cmd/ksh93/sh.1

@@ -6527,10 +6527,7 @@ A
 .BI %( date-format )T
 format can be used to treat an argument as a date/time string
 and to format the date/time according to the
-.I date-format\^
-as defined for the
-.BR date (1)
-command.
+.IR date-format .
 .TP
 .B %Z
 A

src/cmd/ksh93/tests/builtins.sh

@@ -749,5 +749,12 @@ unset foo
 	test_func
 )
 
+# ======
+# Test the output of nonstandard date formats with 'printf %T'
+[[ $(printf '%(%l)T') == $(printf '%(%_I)T') ]] || err_exit 'date format %l is not the same as %_I'
+[[ $(printf '%(%k)T') == $(printf '%(%_H)T') ]] || err_exit 'date format %k is not the same as %_H'
+[[ $(printf '%(%f)T') == $(printf '%(%Y.%m.%d-%H:%M:%S)T') ]] || err_exit 'date format %f is not the same as %Y.%m.%d-%H:%M:%S'
+[[ $(printf '%(%q)T') == $(printf '%(%Qz)T') ]] && err_exit 'date format %q is the same as %Qz'
+
 # ======
 exit $((Errors<125?Errors:125))