Push some time conversions down

Push some time conversions down

* src/timefns.c: Push some time conversions down to lower level fns.
This is a win in its own right and should allow for further speedups.
(lisp_to_timespec): Remove; this convenience function is no longer
needed now that there would be only one caller.  Remaining caller
changed to use definiens.
(enum cform): New constant CFORM_TIMESPEC.  Also, CFORM_SECS_ONLY
now generates a struct timespec instead of a struct lisp_time.
(union c_time.ts): New member.
(decode_ticks_hz): Handle new struct timespec cases.
(decode_float_time, lisp_time_struct): New arg cform, and return
union c_time rather than struct lisp_time.  All callers changed.
(list4_to_timespec, lisp_time_argument, lisp_seconds_argument):
Let lower-level function do the conversion, to allow for better
optimization.

(cherry picked from commit 34bde2f790decfff61f03c5ab92ad0436958bacf)