kmscon: allow paths with --vt=<xy>

kmscon: allow paths with --vt=<xy>

The getty variants out there (including agetty) require an relative path
to the /dev directory as argument. This is really odd but we want to be
backwards-compatible to them so we allow this, too.

--vt now accepts:
  * A positive number which is internally converted into /dev/ttyXY
  * A string that does not start with '/' or '.' which is interpreted
    relative to /dev as /dev/%s
  * Everything else is interpreted as path

This option still selects only the TTY on seat0. On all other seats we do
not use controlling TTYs.

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