adduser: Additional variable fixes.

One function used _input without making it local, causing its value to
leak into other functions. This broke the -f case. Fix that instance
and initialize all local variables that weren't already initialized.

Fixes: 170d0882903e
MFC after: 3 days
Reviewed by: karels
Differential Revision: https://reviews.freebsd.org/D45204

(cherry picked from commit 46a619c6d4cfc662069950baca16ec2a3b18b8a5)