More fixes to stdout and stderr. When the backlog on either clears, call

the backend's unthrottle function.  If we don't, we'll deadlock.  While
we're here, also pump as much data as possible out during each call to
try_output(), rather than restricting ourselves to a single call to
write().

git-svn-id: svn://svn.tartarus.org/sgt/putty@7755 cda61777-01e9-0310-a592-d414129be87e