Add LC_TIME to the supported locale variables (#257)

The current version of 93u+m does not have proper support for the
LC_TIME variable. Setting LC_TIME has no effect on printf %T, and
if the locale is invalid no error message is shown:
    $ LC_TIME=ja_JP.UTF-8
    $ printf '%T\n' now
    Wed Apr  7 15:18:13 PDT 2021
    $ LC_TIME=invalid.locale
    $ # No error message

src/cmd/ksh93/data/variables.c,
src/cmd/ksh93/include/variables.h,
src/cmd/ksh93/sh/init.c:
- Add support for the $LC_TIME variable. ksh93v- attempted to add
  support for LC_TIME, but the patch from that version was extended
  because the variable still didn't function correctly.

src/cmd/ksh93/tests/variables.sh:
- Add LC_TIME to the regression tests for LC_* variables.

NEWS

@@ -3,6 +3,11 @@ For full details, see the git log at: https://github.com/ksh93/ksh
 
 Any uppercase BUG_* names are modernish shell bug IDs.
 
+2021-04-07:
+
+- The $LC_TIME variable is now recognized by ksh and if set to an invalid
+  locale will show an error.
+
 2021-04-05:
 
 - Fixed a regression, introduced in ksh 93t+ 2009-07-31, that caused a command