adduser: Fix confusion between uclass and _class.

This caused adduser to produce an invalid pw(8) command line. Due to
bugs in pw(8), the command line was silently accepted and led to the
user being created, but locked out and with no home directory.

Also fix the default value for the “Another user?” prompt.

Fixes: 170d0882903e
MFC after: 3 days
Reviewed by: karels, allanjude
Differential Revision: https://reviews.freebsd.org/D45098