Implement per-jail fstab(5) files. Here's a rc.conf sample using
this feature for a jail named foo :

jail_foo_mount_enable="YES"
jail_foo_fstab="/etc/fstab.foo"

The second line is actually useless, since the code defaults to
using "/etc/fstab.$jailname" as the fstab file if none is specified.

MFC after: 3 days
Submitted by: Jeremie Le Hen <jeremie@le-hen.org>