Print deprecation notice when running SUID

SUID privilege drop is needed for the "builtin"-backend of libseat,
which copied our old "direct" backend behavior for the sake of
compatibility and ease of transition.

libseat now has a better alternative in the form of seatd-launch. It
uses the normal seatd daemon and libseat backend and takes care of SUID
for us.

Add a soft deprecation warning to highlight our future intent of
removing this code. The deprecation cycle is needed to avoid surprises
when sway no longer drops privileges.

sway/main.c

@@ -153,6 +153,9 @@ static void log_kernel(void) {
 
 static bool drop_permissions(void) {
 	if (getuid() != geteuid() || getgid() != getegid()) {
+		sway_log(SWAY_ERROR, "!!! DEPRECATION WARNING: "
+			"SUID privilege drop will be removed in a future release, please migrate to seatd-launch");
+
 		// Set the gid and uid in the correct order.
 		if (setgid(getgid()) != 0) {
 			sway_log(SWAY_ERROR, "Unable to drop root group, refusing to start");