rs: Use getopt() and strtol() instead of mannual parsing.

This uses the "::" extension to getopt() to handle options which take
an optional argument.

The updated flag tests were all wrong before and only passed because
the manual parser failed to raise errors when a required argument was
missing. The invalid argument test now gets a better error message.

Reviewed by: brooks, imp, emaste
Differential Revision: https://reviews.freebsd.org/D36834