specs: add argument checking to Lua language posix functions.

* specs/posix_spec.yaml (creat, getgroup, spawn, pipeline)
(pipeline_iterator, pipeline_slurp, euidaccess, timeradd)
(timercmp, timersub): Add basic argument checking behaviour
specifications.
* lib/posix.lua.in: Move submodule imports to the start, so they
are available to local Lua functions.
Cache import table lookups into upvalues for fast access.
(creat, getgroup, spawn, pipeline, pipeline_iterator)
(pipeline_slurp, euidaccess, timeradd, timercmp, timersub):
Use cached upvalues everywhere.
Split argument type checking into wrapper function which is not
used when global `_DEBUG` is false.

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