This makes the time type unambiguous, and we can use G_TYPE_INT64 for it
in the divelist too.
It also implements a portable (and thread-safe) "utc_mkdate()" function
that acts kind of like gmtime_r(), but using the 64-bit timestamp_t. It
matches our original "utc_mktime()".
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
