MFH: Always dismount filesystems and try cleaning up when exiting and in jail -k

This fixes a long-standing issue in 3.0.x where interrupting
during initial reference jail creation/mount would leave the
/usr/src nullfs mounted and other various mounts lingering,
as well as the build/ dir having stale data.

In jail_stop(), ignore all errors and keep going. We want everything
possible to be cleaned up.