Bring in timespce_get form NetBSD.

Bring in timespce_get form NetBSD.

Bring in the functionality for timespec_get from NetBSD. I've lightly
edited the .c file to remove _DIAGASSERT because FreeBSD doesn't have
that functionality and the typical #define'ing it to assert isn't
right here. The man page is verbatim from NetBSD, but will be revised
as part of a larger cleanup of the time man pages (they are
inconsistent and vague in all the wrong places).

Differential Review: https://reviews.freebsd.org/D16649