Ignore unspecified type session(s) when look through all active sessions

With modern systemd versions, there is a manager class session,
which is of type "unspecified", for each logged-in / lingering
user. Such session should be ignored when attempt to find an
appropriate session from all (non-seated) sessions, otherwise it
will prevent XDG_SESSION_TYPE (and its "x11" fallback) from being
leveraged.

src/platform/linux.rs

@@ -175,7 +175,7 @@ fn _get_values_of_seat0(indices: &[usize], ignore_gdm_wayland: bool) -> Vec<Stri
                             continue;
                         }
                     }
-                    if d == "tty" {
+                    if d == "tty" || d == "unspecified" {
                         continue;
                     }
                     return line_values(indices, line);