expand checkuser() to support the propagation of error codes back to

expand checkuser() to support the propagation of error codes back to
the caller.  Currently, checkuser() does not differentiate between the
failure to open the file and the absence of a user in the file.  Check
to see if there was an error opening the file.  If there are any errors,
terminate the connection.  Currently, the only exception to this rule
is ENOENT, since there is nothing that says the /etc/ftpuser
and /etc/ftpchroot has to exist.

MFC after:	3 weeks