compat: support xpcall argument passing in Lua 5.1.

* lib/specl/compat.lua (xpcall): If Lua's xpcall does not pass
additional arguments through, pack everything into a closure that
unpacks the arguments for the protected function.
* lib/specl/inprocess.lua: Import xpcall.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specs: luajit error output does not include error line number.

* specs/inprocess_spec.yaml (it captures raised errors): For
compatibility with luajit, don't require error line numbers.

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