rc.d/jail: avoid misinterpreting expr arguments

(Due to some misconfiguration) I ended up with _mask set to
"-v<something>", and /etc/rc.d/jail then failed with
"expr: illegal option -- v".

Use "expr --" so that variable content is never interpreted as an
option.

Reviewed by: jamie
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D14535