text: Title: BackdoorServer doesn't work anymore with AF_UNIX listeners
  Body: * gevent version: 20.12.1 from pypi
  * Python version: 2.7.18 from the official Fedora repos.
  * Operating System: Fedora 5.8.10-200.fc32.x86_64
  
  ### Description:
  
  `gevent.backdoor.BackdoorServer()`  doesn't work when given an `AF_UNIX` listener. It assumes the listener is `AF_INET` and tries to do a `conn.setsockopt(socket.SOL_TCP, socket.TCP_NODELAY, True)` in `BackdoorServer.handle()`.
  
  Previously, on an older gevent version unix sockets worked fine. In some cases they're preferable over TCP on localhost, because basic authentication can be done with the socket file owner/permissions.
  
  We can probably check the socket type in `handle()` with `getsockopt(SO_DOMAIN)` or something. Thoughts?