chroot: add DragonFly BSD support

chroot: add DragonFly BSD support

The support is basically the same as the FreeBSD support except
that DragonFly BSD uses devfsctl to modify devfs mounts and there's
no system-provided devfs ruleset for chroots so we must provide one.

Additionally, DragonFly BSD does not have any netcat port available
with support for the -U option.  This is a problem even for non-chroot
installations of Girocco.  Therefore we provide src/dragonfly with a
suitable (OpenBSD) version of netcat (nc.openbsd) for DragonFly BSD.o

Various comments are added to note the new additions and subtle
differences (such as the loopback fs type being null instead of nullfs).

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>