* lib-src/b2m.c (main): Don't include <limits.h>.

* lib-src/b2m.c (main): Don't include <limits.h>.
(TM_YEAR_BASE): New macro.
(TM_YEAR_IN_ASCTIME_RANGE): Don't define if already defined, so
that s/ files can override this.  Use the more-conservative range
1000-9999.
(main): Check for asctime returning NULL.
* lib-src/fakemail.c: Likewise.
* src/editfns.c (TM_YEAR_IN_ASCTIME_RANGE): New macro, identical to
../lib-src/b2m.c and ../lib-src/editfns.c.
(Fcurrent_time_string): Use it.
Document that the year might not consume 4 columns if it's outside
the range 1000-9999.
Check for asctime failure.
Don't assume that the output string length is always exactly 24.