procutil: introduce context-manager interface for protect/restorestdio The code looks slightly cleaner since it was pretty easy to pass arguments in wrong order.

procutil: move protectio/restoreio from commandserver Some variants of this will be useful for stdio-based servers such as sshserver.

commandserver: rewrite protectio/restoreio to not depend on ui Prepares for porting to utils.procutil, in which ui shouldn't be known. ui.flush() is replaced with ui.fout.flush() since ui.ferr wasn't involved.