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