adduser: Overhaul.

Most importantly:

• Make local variables local.
• Use $() instead of backticks.
• Avoid unsafe use of -a and -o operators in test expressions.
• Remove a hack intended to ease the transition from Perl 22 years ago.

MFC after: 1 week
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44863