loader: Simplify the loader.has_command

luaL_checkstring already checks for the right number of
arguments. There's no need to do that by hand here. Now an exception
will be thrown like any other function with the wrong args. Also,
push a boolean instead of an int.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D43820

(cherry picked from commit 7fc95c31f007ef01c53aa5f9d8802e9579f408ee)