Make use_yp() smarter about figuring out whether a user is local or

Make use_yp() smarter about figuring out whether a user is local or
NIS (or both, or neither). Also add support for -l and -y flags to
force behavior to local or NIS. use_yp() also goes out of its way to
retrieve the correct password database information (local or NIS)
depending on what the situation since getpwent() & co. can't
necessarily be trusted in some cases.

Also document new flags in man page.