There are some deltas between the UN*X epoch and other epochs that are
used in a number of places; put them into a header.
Change-Id: Ia2d9d69b9d91352d730d97d9e4897518635b4861
Reviewed-on: https://code.wireshark.org/review/35895
Petri-Dish: Guy Harris <guy@alum.mit.edu>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <guy@alum.mit.edu>