In timefns.c avoid by-hand overflow checking

In timefns.c avoid by-hand overflow checking

Prefer functions like ckd_add to do overflow checking, instead of
doing it by hand, to simplify and I hope to make things a bit less
error prone.
* src/timefns.c (TIME_T_MIN, TIME_T_MAX): Remove.  All by-hand
overflow checking replaced with calls to ckd_add or ckd_mul.
(s_ns_to_timespec): New static function, that uses ckd_add
instead of by-hand overflow checking.
(ticks_hz_to_timespec): Use it.
(check_tm_member): Use mpz_fits_sint_p and mpz_get_si rather
than mpz_to_intmax and by-hand overflow checking.

(cherry picked from commit 75f53d7c2c1f6706d1d4c1f3eed3aa675464c62f)