bsdinstall: Mount /dev and /packages after using the shell to partition disks

Normally after partitions are created by the installer, the 'mount'
script is used to mount the target disk partitions under /mnt. The
tail end of this script also mounts a couple of additional filesystems
under /mnt so that chrooted programs can work such as devfs and
/packages.

When the "Shell" option is used to permit the user to manually mount
the destination filesystem, the "mount" script is not used as the user
is instructed to mount the target filesystems and construct
/mnt/etc/fstab, etc. However, this means that the user is responsible
for mounting devfs (which is not included in /etc/fstab) and /packages
as well. The help message for the "Shell" option doesn't mention
these requirements, so users may not know to do so. This can lead to
confusing errors as chrooted commands can fail to find needed /dev
entries. For example, running fwget to fetch wireless firmware fails
because /dev/pci doesn't exist.

To make this less painful for users using this option, split out the
bottom half of the 'mount' script that mounts these non-fstab-related
filesystems into a separate 'mount_aux' script. Invoke 'mount_aux'
after using "Shell" to create the filesystem to ensure that these
filesystems are always present.

PR: 290901
Reported by: Peter <freebsd@peterk.org>
Tested by: Peter <freebsd@peterk.org>
Differential Revision: https://reviews.freebsd.org/D53770