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

NEWS

@@ -3,6 +3,17 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2020-07-07:
+
+- Four of the date formats accepted by 'printf %()T' have had their
+  functionality altered to the common behavior of date(1):
+  - '%k' and '%l' print the current hour with blank padding, the former
+    based on a 24-hour clock and the latter a twelve hour clock. These
+    are common extensions present on Linux and *BSD.
+  - '%l' prints the current hour (0-12) with blank padding (GNU and BSD).
+  - '%f' prints a date with the format string '%Y.%m.%d-%H:%M:%S' (BusyBox).
+  - '%q' prints the quarter of the year (GNU).
+
 2020-07-06:
 
 - 'notty' is now written to the ksh auditing file instead of '(null)' if