MFC r335308: bsdconfig: Fix a bug when editing users

MFC r335308: 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>
Sponsored by:	Smule, Inc.

git-svn-id: svn://svn.freebsd.org/base/stable/10@335487 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f