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/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))