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 570121808a76b85b2709502fb15618dd1e5296f1)