* ext/socket/unixsocket.c (unix_send_io): document args
  (unix_recv_io): ditto
* test/socket/test_unix.rb (test_fd_passing_class_mode): added

I was working on these when I encountered the problem in
with BasicSocket.for_fd not handling mode args:
https://bugs.ruby-lang.org/issues/11778

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52922 b2dd03c8-39d4-4d8f-98ff-823fe69b080e