sys.time: wrap gettimeofday properly.

* specs/posix_spec.yaml (gettimeofday): Move from here...
* specs/posix_compat.yaml (gettimeofday): ...to here.
* specs/posix_sys_time_spec.yaml (gettimeofday): Specify correct
thin binding for gettimeofday(2).
* specs/specs.mk (specl_SPECS): Add posix_sys_time_spec.yaml.
* ext/include/_helpers.c (pushtimeval): New function. Push a
struct timeval-like table on top of the stack.
* ext/posix/sys/time.c (Pgettimeofday): Simplify accordingly.
* lib/posix/compat.lua (gettimeofday): Rewrite misnamed table
fields returning API here for backwards compatibility.

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