Fix incorrect use of errno in various libcomm.la places

Fix incorrect use of errno in various libcomm.la places

Fix problems with 'errno' in TcpAcceptor::Listen, Comm::HandleRead, and
Comm::HandleWrite. 'errno' is only valid after a standard library function
returns an error. Also, we must avoid calling out to other functions before
recording the value of 'errno', since they might overwrite it.