These were originally present in the early unshield code but
for some reason they were removed later (this change is based
on the i5comp and i6comp struct layout)

Those struct members have a correct date with the Baldur's Gate
test (likely <IS6?) and GTA San Andreas (IS9)

This also adds a helper function to load the DOS date/time into a tm
structure and an library function to get the ISO 8601 date

This also adds an API function for getting an Unix timestamp
(and updates the man page)