fix(udp): retry send on first EINVAL

Android API level < 26 does not support the `libc::IP_TOS` control message.
`sendmsg` calls with `libc::IP_TOS` return `libc::EINVAL`.

#1516 added a fallback, not setting
`libc::IP_TOS` on consecutive calls to `sendmsg` after a failure with
`libc::EINVAL`. The current datagram would be dropped. Consecutive datagrams
passed to `sendmsg` would succeed as they would be sent without `libc::IP_TOS`
through the fallback.

Instead of dropping the first datagram on `libc::EINVAL`, this commit adds a
retry for it without `libc::IP_TOS`.

This is e.g. relevant for Neqo. When establishing a QUIC connection, dropping
the first datagram [delays connection establishment by
100ms](https://github.com/mozilla/neqo/blob/3001a3a56f2274eaafaa956fb394f0817f526ae7/neqo-transport/src/rtt.rs#L28).
With the retry introduced in this commit, delay due to unsupported
`libc::IP_TOS` should be negligeable.

Closes #1975.

quinn-udp/src/unix.rs

@@ -330,10 +330,20 @@ fn send(
                         }
                     }
 
-                    if e.raw_os_error() == Some(libc::EINVAL) {
-                        // Some arguments to `sendmsg` are not supported.
-                        // Switch to fallback mode.
+                    // Some arguments to `sendmsg` are not supported. Switch to
+                    // fallback mode and retry if we haven't already.
+                    if e.raw_os_error() == Some(libc::EINVAL) && !state.sendmsg_einval() {
                         state.set_sendmsg_einval();
+                        prepare_msg(
+                            transmit,
+                            &dst_addr,
+                            &mut msg_hdr,
+                            &mut iovec,
+                            &mut cmsgs,
+                            encode_src_ip,
+                            state.sendmsg_einval(),
+                        );
+                        continue;
                     }
 
                     // - EMSGSIZE is expected for MTU probes. Future work might be able to avoid