bsdinstall mount: Replace dialog with bsddialog

[asiciliano]

Summary

Replace (LGPL) dialog utility with (BSD-2-Clause) bsddialog utility.

Test Plan

it is just a --msgbox,
anyway bsddialog runs if an error occurs, to cause it I changed the first line:

#TMP_FSTAB=${TMPDIR:-"/tmp"}/bsdinstall-tmp-fstab
TMP_FSTAB="/etc/fstab"

then:

% sh ./mount
./mount: cannot create /etc/fstab: Permission denied
Ctrl-C

┌───────────────────┤Error├───────────────────┐
│ Error mounting partition /:                 │  
│ mount: /dev/ada0p4: Operation not permitted │  
├─────────────────────────────────────────────┤  
│                  [  OK  ]                   │  
└─────────────────────────────────────────────┘