kmscon: introduce new --listen mode

kmscon: introduce new --listen mode

Instead of configuring everything for every kind of situation, we now
split kmscon into two modes: default-mode and listen-mode

In default-mode we run on the given seats once until we encounter a HUP or
until we are closed. It's a perfect replacement for agetty.

In listen-mode, we run only on non-VT seats and provide the full kmscon
functionality. We run as system daemon and wait for new seats and close
seats if they are destroyed. We simply provide a full system-console on
all seats.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>