Add support for TZ="foo" within a date string.

Add support for TZ="foo" within a date string.
Fix some bugs near time_t boundaries.  Reject dates with
out-of-range components, e.g., "Sept 31".
Include <stdlib.h>, "setenv.h", "xalloc.h".
(ISDIGIT_LOCALE): Remove; unused.
Note that the TZ and time functions used here are not reentrant.
(mktime_ok, get_tz): New functions.
(TZBUFSIZE): New constant.
(get_date): Parse leading TZ="foo".  Reject out-of-range components;.
This requires that we sometimes generate our own TZ="XXX..." setting.