timespec: new function current_timespec
* lib/gettime.c (gettime): Prefer clock_gettime to nanotime,
and don’t worry about it failing on a CLOCK_REALTIME arg.
POSIX requires it to succeed and I don’t know of any
counterexamples where the fallbacks would work.
(current_timespec): New function, taken from Emacs. It is more
convenient than gettime, and can help register allocation.
* lib/timespec.h: Include arg-nonnull.h.
(current_timespec): New declaration.
(gettime, settime): Declare args to be nonnull.
* modules/timespec (Depends-on): Add snippet/arg-nonnull.