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)
 			{