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/lib/libast/tm/tmxfmt.c

@@ -271,8 +271,8 @@ tmxfmt(char* buf, size_t len, const char* format, Time_t t)
 		case 'e':       /* blank padded day of month */
 			cp = number(cp, ep, (long)tm->tm_mday, -2, width, pad);
 			continue;
-		case 'f':	/* (AST) OBSOLETE use %Qf */
-			p = "%Qf";
+		case 'f':	/* Output the date in a format compatible with BusyBox %f */
+			p = "%Y.%m.%d-%H:%M:%S";
 			goto push;
 		case 'F':	/* ISO 8601:2000 standard date format */
 			p = "%Y-%m-%d";
@@ -316,8 +316,8 @@ tmxfmt(char* buf, size_t len, const char* format, Time_t t)
 		case 'J':	/* Julian date (0 offset) */
 			cp = number(cp, ep, (long)tm->tm_yday, 3, width, pad);
 			continue;
-		case 'k':	/* (AST) OBSOLETE use %QD */
-			p = "%QD";
+		case 'k':	/* hour (0 - 23) with blank padding */
+			p = "%_H";
 			goto push;
 		case 'K':	/* (AST) largest to smallest */
 			switch (alt)
@@ -333,8 +333,8 @@ tmxfmt(char* buf, size_t len, const char* format, Time_t t)
 				break;
 			}
 			goto push;
-		case 'l':	/* (AST) OBSOLETE use %QL */
-			p = "%QL";
+		case 'l':	/* hour (0 - 12) with blank padding */
+			p = "%_I";
 			goto push;
 		case 'L':	/* (AST) OBSOLETE use %Ql */
 			p = "%Ql";
@@ -364,9 +364,9 @@ tmxfmt(char* buf, size_t len, const char* format, Time_t t)
 			while (cp < ep && (n = *p++))
 				*cp++ = isupper(n) ? tolower(n) : n;
 			continue;
-		case 'q':	/* (AST) OBSOLETE use %Qz */
-			p = "%Qz";
-			goto push;
+		case 'q':	/* quarter of the year (1-4) */
+			cp = number(cp, ep, (long)(tm->tm_mon / 3) + 1, 0, width, pad);
+			continue;
 		case 'Q':	/* (AST) %Q<alpha> or %Q<delim>recent<delim>distant<delim> */
 			if (c = *format)
 			{

src/lib/libcmd/date.c

@@ -95,26 +95,26 @@ USAGE_LICENSE
 "		[+d?day of month number]"
 "		[+D?date as \amm/dd/yy\a]"
 "		[+e?blank padded day of month number]"
-"		[+f?locale default override date format]"
+"		[+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]"
+"		[+H?24-hour clock hour, zero-padded]"
 "		[+i?international \bdate\b(1) date with time zone type name]"
-"		[+I?12-hour clock hour]"
+"		[+I?12-hour clock hour, zero-padded]"
 "		[+j?1-offset Julian date]"
 "		[+J?0-offset Julian date]"
-"		[+k?\bdate\b(1) style 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?\bls\b(1) \b-l\b date; equivalent to \b%Q/%g/%G/\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?time zone type name (nation code)]"
+"		[+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]"