Add macros to initialize nstime_t values and use them.

Add macros to initialize nstime_t values and use them.

This allows code to initialize them without having to know the details
of the structure; the initializes should, and will, be changed if the
members of the structure are changed.

Change-Id: I93e6ebfcde9ceca17df696fcba4e8410c5afb175
Reviewed-on: https://code.wireshark.org/review/28501
Reviewed-by: Guy Harris <guy@alum.mit.edu>