rs: Fix various harmless warnings.
- Add /* FALLTHROUGH */ comments for intentional fall throughs in getargs().
- Remove id strings to quiet -Wunused-const-variable warnings from GCC.
- While here, remove __FBSDID.
Reviewed by: brooks
Differential Revision: https://reviews.freebsd.org/D36830
(cherry picked from commit bb31e1bbf2ade7abd38eb29391e1ba0718723480)