better support for 700+ connections

when there was more than ~700 active connections,
* sendfile (non-https downloads) could fail
* mdns and ssdp could fail to reinitialize on network changes

...because `select` can't handle FDs higher than 512 on windows
(1024 on linux/macos), so prefer `poll` where possible (linux/macos)

but apple keeps breaking and unbreaking `poll` in macos,
so use `--no-poll` if necessary to force `select` instead

copyparty/mdns.py

@@ -292,6 +292,22 @@ class MDNS(MCast):
     def run2(self) -> None:
         last_hop = time.time()
         ihop = self.args.mc_hop
+
+        try:
+            if self.args.no_poll:
+                raise Exception()
+            fd2sck = {}
+            srvpoll = select.poll()
+            for sck in self.srv:
+                fd = sck.fileno()
+                fd2sck[fd] = sck
+                srvpoll.register(fd, select.POLLIN)
+        except Exception as ex:
+            srvpoll = None
+            if not self.args.no_poll:
+                t = "WARNING: failed to poll(), will use select() instead: %r"
+                self.log(t % (ex,), 3)
+
         while self.running:
             timeout = (
                 0.02 + random.random() * 0.07
@@ -300,8 +316,13 @@ class MDNS(MCast):
                 if self.unsolicited
                 else (last_hop + ihop if ihop else 180)
             )
-            rdy = select.select(self.srv, [], [], timeout)
-            rx: list[socket.socket] = rdy[0]  # type: ignore
+            if srvpoll:
+                pr = srvpoll.poll(timeout * 1000)
+                rx = [fd2sck[x[0]] for x in pr if x[1] & select.POLLIN]
+            else:
+                rdy = select.select(self.srv, [], [], timeout)
+                rx: list[socket.socket] = rdy[0]  # type: ignore
+
             self.rx4.cln()
             self.rx6.cln()
             buf = b""