signal: modernize posix.signal.

Close #161.
* specs/posix_spec.yaml (kill, killpg, raise, signal): Move from
here...
* specs/posix_signal_spec.yaml (kill, killpg, raise, signal):
New file.  ...to here.
* specs/specs.mk (specl_SPECS): Add specs/posix_signal_spec,yaml.
* examples/signal.lua: Rewrite in modern style.
* ext/posix/signal.c (Psignal): Update LDocs.
(luaopen_posix_signal): Add SIG_DFL and SIG_IGN constants.
* NEWS: Update.

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