Fix process-attributes time precision on BSD

Fix process-attributes time precision on BSD

* src/sysdep.c (timeval_to_timespec) [__FreeBSD__ || DARWIN_OS]:
Remove; no longer needed.
(make_lisp_s_us) [__FreeBSD__ || DARWIN_OS || __OpenBSD__]:
New function.
(make_lisp_timeval) [__FreeBSD__ || DARWIN_OS || __OpenBSD__]:
Rework in terms of make_lisp_s_us.
(system_process_attributes) [__FreeBSD__ || DARWIN_OS ||
__OpenBSD__]: Simplify by using the above.  This fixes some minor
problems where timestamps promised more precision than was
actually available.