* ext/socket/raddrinfo.c (rsock_unixpath_len, init_unix_addrinfo),
  ext/socket/unixsocket.c (unixsock_connect_internal,
  rsock_init_unixsock): calculate the correct address length of
  an abstract socket.  Without this fix, sizeof(struct sockaddr_un)
  is specified as the length of an abstract socket for bind(2) or
  connect(2), so the address of the socket is filled with extra NUL
  characters.  See unix(7) for details.

* ext/socket/lib/socket.rb (unix_server_socket): don't access the
  file system if the platform is Linux and path starts with NUL,
  which means that the socket is an abstract socket.

* test/socket/test_unix.rb: related test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38939 b2dd03c8-39d4-4d8f-98ff-823fe69b080e