gdm-session: require a password for for remote logins

Many remote services refuse passwordless logins, so administrators may find it
surprising that the login screen over XDMCP doesn't require a password on the
account.
This commit toggles to that default.

Note, this change doesn't really make XDMCP more secure. It is inherently
insecure over an untrusted network, since it's a completely plain text
protocol.

https://bugzilla.gnome.org/show_bug.cgi?id=764669