cmdserver: write channel header and payload by a single write() call This makes a channeledoutput thread-safe as long as the underlying fwrite() is thread-safe. Both POSIX and Windows implementations are documented as MT-safe. MT-safety is necessary to use ui.fout and ui.ferr in hgweb.

histedit: use ui.fin to read commands from stdin stdin is wrapped by channeledinput in command-server session.