libc/stdtime: Set errno to EOVERFLOW when there is an overflow.

libc/stdtime: Set errno to EOVERFLOW when there is an overflow.

This fixes various issues such as gmtime() returning NULL on an
out-of-bounds time_t but not setting errno, as POSIX requires.

Also, in ctime() and ctime_r(), check the result of localtime()
before passing it to asctime{,_r}().

See:
https://github.com/eggert/tz/commit/4d306b3a17ce5ce0b33a73a90dc713d3601ea89a