jls: minor simplification to arg handling

It's easier to reason about the state of argc/argv if we just augment
them by optind after our getopt() loop.

No functional change, but this sets the stage for another change to add
a -c mode to (c)heck for the existence of a jail quietly without
the caller having to worry about spurious output.

Reviewed by: jamie
Differential Revision: https://reviews.freebsd.org/D51540