set unix socket permissions during binding of the UNIX-socket #426

Unfortunatelly this is not so easy.

1. Safe, but (seems?) undocumented way in Linux -- is to call os.fchmod(socket.fileno()) BEFORE bind() + set umask to proper value. Since umask is not thread safe (affect all threads) that action should be done in fork()...
2. Portable way -- is to call os.chmod(path) after binding. But this will leave socket with wrong permissions during small amount of time.

Socket write permissions are required in order unprivileged process to connect to it.