bsdconfig: Fix a bug when editing users

The usermgmt API was stomping on a global ($user_gid to be specific)
so things would appear to work fine until you tried to make a second
pass into the API with the now-tainted variable contents.

Fixed by localizing menu-specific contents as to not leak outside API.

PR: bin/208774
Reported by: Martin Waschbuesch <martin@waschbuesch.de>
MFC after: 1 week
X-MFC-to: stable/11, stable/10
Sponsored by: Smule, Inc.