chgserver: backport py3 buffered I/O workarounds from procutil
I've recently switched to new machine and I found chg's stdout is fully buffered. Even though chg server is a daemon process, it inherits the environment where the chg client originally forked the server. This means the server's stdout might have been wrapped by LineBufferedWrapper. That's why we need to do wrap/unwrap in both ways. The "if" condition in _restoreio() looks weird, but I'm not willing to clean things up because stdio behavior is fundamentally different between py2 and py3, and py2 support will be dropped anyway.