Remove broken unknown arguments check and fix usage

If getopts comes across an unknown argument, $arg it set to '?' and
$OPTARG is unset. Therefore the getopts line detecting unknown arguments
doesn't work. Arguments to pass to makepkg are already handled by
passing all the aguments after the end-of-options marker (--), but this
wasn't documented in the usage text.

Signed-off-by: Pierre Schmitz <pierre@archlinux.de>