time: bind nanosleep minimally.

Close #162.
* specs/posix_spec.yaml (nanosleep): Move from here...
* specs/posix_compat_spec.yaml (nanosleep): ...to here.
* specs/posix_time_spec.yaml (nanosleep): Specify behaviour of
minimal binding.
* ext/posix/time.c (totimespec, pushtimespec): New functions.
Convert between C struct timespec and Lua PosixTimespec table.
(nanosleep): Simplify accordingly.
* lib/posix/compat.lua (nanosleep): Reimplement old API over new
minimal binding.
* NEWS: Update.

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