x11/sddm: add workaround for Wayland sessions.

Mimic seatd and put the underlying tty in raw mode
to prevent killing Wayland sessions on CTRL+C.

Upstream reports:
KDE: https://bugs.kde.org/505552
SDDM: https://github.com/sddm/sddm/issues/1986

PR: 286592
Submitted by: jsm, jhale