posix: argument errors overhaul.

* specs/spec_helper.lua (bind, init): Remove.
* specs/lposix_spec.yaml, specs/posix_spec.yaml: Specify argument
error diagnostics for all APIs.
* ext/posix/posix.c: Use checknargs, or other means to diagnose
argument type and arity errors for all APIs.
* NEWS: Update.

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