login: give session ownership of VT

The tty associated with a VT should be owned by the owner of the session
running on the VT. This is important for supporting a socket activated X
server, since the X server will open the tty itself.

This commit makes sure to chown the tty any time a session is
created and and chown it back to root any time the session
is removed. This behavior is copied from /bin/login.
login: rename session_mute_vt() to session_prepare_vt()

This function is no longer just about muteing the VT. We do all kinds of
VT setup for sessions using the controller-API. Rename the function to
something more appropriate.