login: rerun vconsole-setup when switching from vgacon to fbcon

The initialization performed by systemd-vconsole-setup is reset
when changing console drivers (say from vgacon to fbcon), so we
need to run it in that case.

See
http://lists.freedesktop.org/archives/systemd-devel/2014-October/023919.html
http://lists.freedesktop.org/archives/systemd-devel/2014-October/024423.html
http://lists.freedesktop.org/archives/systemd-devel/2014-November/024881.html

This commit adds a udev rule to make systemd-vconsole-setup get run when
the fbcon device becomes available.

(david: moved into new file 90-vconsole.rules instead of 71-seats.rules;
        build-failures are on me, not on Ray)