bsdinstall fetchmissingdists: Replace dialog with bsddialog

[asiciliano]

Summary

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

Test Plan

% bsddialog --backtitle "FreeBSD Installer" --title "Warning" --msgbox "Manifest not found on local disk and will be fetched from an unverified source. This is a potential security risk. If you do not wish to proceed, press control-C now." 0 0

┌───────────────────────────────────────┤Warning├───────────────────────────────────────┐
│ Manifest not found on local disk and will be fetched from an unverified source. This  │  
│ is a potential security risk. If you do not wish to proceed, press control-C now.     │  
├───────────────────────────────────────────────────────────────────────────────────────┤  
│                                       [  OK  ]                                        │  
└───────────────────────────────────────────────────────────────────────────────────────┘

for error() to add in script

error "TEST test Test"

┌─────┤Error├────┐
│ TEST test Test │  
├────────────────┤  
│    [  OK  ]    │  
└────────────────┘