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)