time-util: rework localtime_or_gmtime() into localtime_or_gmtime_usec()

time-util: rework localtime_or_gmtime() into localtime_or_gmtime_usec()

We typically want to deal in usec_t, hence let's change the prototype
accordingly, and do proper range checks. Also, make sure are not
confused by negative times.

Do something similar for mktime_or_timegm().

This is a more comprehensive alternative to #34065

Replaces: #34065