emacs: show: stop stderr appearing in buffer

emacs: show: stop stderr appearing in buffer

In emacs 24.3+ the stdout/stderr from externally displaying an
attachment gets inserted into the show buffer. This is caused by
changes in mm-display-external in mm-decode.el.

Ideally, we would put this output in the notmuch errors buffer but the
handler is called asynchronously so we don't know when the output will
appear. Thus if we put it straight into the errors buffer it could get
interleaved with other errors. Also we can't easily tell when we
have got all the error output so can't wait until the process is complete.

One solution would be to create a new buffer for the stderr of each
attachment viewed. Again, since we can't tell when the process has
finished, we can't close these buffers automatically so this will
leave lots of buffers around.

Thus we add a debug variable notmuch-show-attachment-debug: it this is
non-nil we create a new buffer for each viewer; if this variable is
nil we just use a temp buffer which means all error output is
discarded (this is the same behaviour as with emacs pre 24.3).