MFS jail: Change both root and working directories in jail_attach(2)
jail_attach(2) performs an internal chroot operation, leaving it up to
the calling process to assure the working directory is inside the jail.
Add a matching internal chdir operation to the jail's root. Also
ignore kern.chroot_allow_open_directories, and always disallow the
operation if there are any directory descriptors open.
Reported by: mjg
Approved by: re (gjb), markj, kib