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

(cherry picked from commit b7d53d8438e66f84768f28cbccc92a04d5df76b8)