bsdinstall checksum: Replace dialog with bsddialog

[asiciliano]

Summary

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

Test Plan

mixedgauge: testes by distextract and distfetch.
msgbox: is trivial.

However to be sure, comment first line
#test -f $BSDINSTALL_DISTDIR/MANIFEST || exit 0
then:
% env DISTRIBUTIONS="One Two Tree" ./checksum

┌─┤Checksum Verification├─┐
│ One     [ In Progress ] │  
│ Two     [   Pending   ] │  
│ Tree    [   Pending   ] │  
│                         │  
│ Verifying checksums of  │  
│ selected distributions. │  
│                         │  
│  ┌─Overall Progress──┐  │  
│  │         0%        │  │  
│  └───────────────────┘  │  
└─────────────────────────┘

sha256: /One: No such file or directory
awk: can't open file /MANIFEST
source line number 7

┌─────────────────────────────────────────┤Error├────────────────────────────────────────┐
│ The checksum for One does not match. It may have become corrupted, or it may be from a │  
│ newer version of FreeBSD. Please check for a newer snapshot.                           │  
├────────────────────────────────────────────────────────────────────────────────────────┤  
│                                        [  OK  ]                                        │  
└────────────────────────────────────────────────────────────────────────────────────────┘