time: bind clock_getres and clock_gettime minimally.

Close #159.
* specs/posix_time_spec.yaml (clock_getres, clock_gettime): Adjust
to specify passing of integer constants and return PosixTimespec
tables.
* specs/posix_compat_spec.yaml (clock_getres, clock_gettime):
Specify backwards compatible behaviours accepting optional name
and returning separate sec, nanosec results.
* ext/posix/time.c (get_clk_id_const): Remove.
(clock_getres, clock_gettime): Simplify.
* lib/posix/compat.lua (clock_getres, clock_gettime): Reimplement
old API over new minimal binding.
* NEWS: Update