jls: add a -c mode to check for a jail's existence

This is intended to be primarily used in scripts that want to check if
a jail exists before taking some action -- for instance, the jail(8)
test cleanup functions that want to remove the jail if it still exists.
Having a mode that limits its output for their usage is useful both
for cleaner scripts and also to avoid masking real problems.

Reviewed by: jamie
Differential Revision: https://reviews.freebsd.org/D51541