refactor: separate private argument checking helpers.

* lib/posix/compat.lua (argerror, argtypeerror, badoption)
(checkint, checkselection, checkstring, checktable, optint)
(optstring, opttable, toomanyargerror): Move from here...
* lib/posix/_argcheck.lua (argerror, argtypeerror, badoption)
(checkint, checkselection, checkstring, checktable, optint)
(optstring, opttable, toomanyargerror): New file.  ...to here.
* local.mk (dist_luaposix_DATA): Add lib/posix/_argcheck.lua.

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