server: make sure accepted UNIX sockets are blocking

It was a problem on Mac OS X, where sometimes we got EAGAIN
errors from calling socket.recv() on them.

trezor_agent/server.py

@@ -63,6 +63,7 @@ def server_thread(server, handler, quit_event):
 
     def accept_connection():
         conn, _ = server.accept()
+        conn.settimeout(None)
         return conn
 
     while True:

trezor_agent/tests/test_server.py

@@ -32,6 +32,9 @@ class FakeSocket(object):
     def close(self):
         pass
 
+    def settimeout(self, value):
+        pass
+
 
 def test_handle():
     handler = protocol.Handler(keys=[], signer=None)