MFC jail: Change both root and working directories in jail_attach(2)

MFC 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.

Approved by:	so
Security:	CVE-2020-25582
Security:	FreeBSD-SA-21:05.jail_chdir
Reported by:    mjg
Approved by:    markj, kib

(cherry picked from commit d4380c0cdd0517dc038403dd5c99242ce78bdeb5)
(cherry picked from commit ca9ab8ea17748a1758701fde262cb272fb757989)