Mount and unmount devfs around calls to add packages.

pkg now uses /dev/null for some of its operations. NanoBSD's packaging
stuff didn't mount that for the chroot it ran in, so any config that
added packages would see the error:
pkg: Cannot open /dev/null:No such file or directory
when trying to actually add those packages. It's easy enough for
nanobsd to mount /dev and it won't hurt anything that was already
working and may help things that weren't (like this). I moved the
mount/unmount pair to be in the right push/pop order from the
submitted patch.

PR: 238727
Submitted by: mike tancsa
Tested by: Karl Denninger

(cherry picked from commit 804b78634d543667a37ac459e692fe42c9dd30e8)