Further simplify arguments to init.

With the removal of BOOTCDROM and fastboot support, this code always
passed "-s" or "--". The latter simply terminates getopt(3) processing
in init so we only need to pass "-s" in the single user case, or nothing
in other cases.

The passing of "--" seems to have been done to ensure that the number of
arguments passed to init was always the same and thus that argc was the
same.

Also GC the write-only variable pathlen (not in reviewed version).

Reviewed by: kib, jhb
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D18441