timespec: new function make_timespec, and new constants

timespec: new function make_timespec, and new constants

* lib/timespec.h: Incorporate recent changes on the Emacs trunk.
(TIMESPEC_RESOLUTION, LOG10_TIMESPEC_RESOLUTION): New constants.
(make_timespec): New function.
* lib/dtotimespec.c (dtotimespec):
* lib/timespec-add.c (timespec_add):
* lib/timespec-sub.c (timespec_sub):
* lib/utimens.c (validate_timespec):
* lib/utimensat.c (rpl_utimensat):
Use these new constants and functions.