common: Add new function gnupg_gmtime.

* common/gettime.c (gnupg_gmtime): New.
(gnupg_get_isotime): Use it.  Also take care of an gmtime_t returning
an error.

The fix in gnupg_get_isotime is only to cover up a theoretical broken
time (e.g. a value of (time_t)(-2) which is not mapped beyond 2038 on
32 bit systems).

• Signed-off-by: Werner Koch <wk@gnupg.org>