mfc jail: handle jailsys parameters in modification permission test

Avoid a null dereference when a value-less jailsys parameter is passed
to "jail -m". There was already code to handle boolean parameters,
but in reality any parameter could be passed without a value.

PR: 262471
Reported by: jcaplan at blackberry.com

(cherry picked from commit 8f1543785f77086494c73310ba8f5d09b61ff7eb)