times: bind times minimally.

Close #165.
* specs/posix_spec.yaml (times): Move from here...
* specs/posix_compat_spec.yaml (times): ...to here.
* specs/posix_sys_times_spec.yaml (times): New file. Specify
minimal binding of times(2).
* specs/specs.mk (specl_SPECS): Add
specs/posix_sys_times_spec.yaml.
* ext/posix/sys/times.c (struct mytimes, Ftimes, Stimes): Remove.
(Ptimes): Simplify accordingly.
* lib/posix/compat.lua (times): Reimplement legacy multi-string
or table io/API over minimal C binding.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>