Remove the need for rust nightly

My pidfd implmentation is simple enough, and Option.contains() is not
worth requiring nightly.

util/src/lib.rs

@@ -12,6 +12,10 @@ pub fn init_logging(env_name: &str) {
         .init();
 }
 
+pub fn opt_contains<T: PartialEq>(o: &Option<T>, v: &T) -> bool {
+    o.as_ref().filter(|ov| ov == &v).is_some()
+}
+
 pub fn create_atom(conn: &xcb::Connection, name: &[u8]) -> xcb::Result<x::Atom> {
     let cookie = conn.send_request(&x::InternAtom {
         only_if_exists: false,