main: replace unused --seat argument with --seats

main: replace unused --seat argument with --seats

The old --seat argument was no implemented, so replace it with --seats
which accepts a list of seats where kmscon runs on. Each seat is separate
from the others but kmscon allows providing terminal services to multiple
users/seats in a single process sharing resources like fonts and
renderers.

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