sys: new submodule for non-POSIX apis.

Close #73
* lib/posix.lua (list, euidaccess, pipeline, pipeline_iterator)
(pipeline_slurp, spawn, timeradd, timersub): Move from here...
* lib/posix/sys.lua (list, euidaccess, pipeline)
(pipeline_iterator, pipeline_slurp, spawn, timeradd, timersub):
New file. ...to here.
* lib/posix.lua (__index): Attempt to load a suitably named
submodule to satisfy missing keys.
* NEWS: Update.

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