on: handle remote stdout and stderr via mux

on: handle remote stdout and stderr via mux

Previously, when running "bup on" (i.e. when in BUP_SERVER_REVERSE
mode), bup would redirect the remote command's stdout to stderr because
stderr is the only remaining avenue back to the local console.  It's the
only avenue because in reverse mode, stdout (and stdin) are connected
back to a local "bup server" instance (hence the "reverse").

Of course that makes it impossible to reliably capture the
non-diagnostic output from the remote commands, i.e.

  commit_id="$(bup on HOST save -t ...)"

To fix that, in the remote "bup on--server" multiplex the stdout and
stderr from all "bup on" subcommands with "bup mux", and then
demultiplex those streams back to the local stdout and stderr via
DemuxConn() in the receiving "bup on".

Thanks to Alexander Barton for pointing out an error in a previous
version of this commit message, and thanks to Gabriel Filion for
pointing out that we could use the existing mux infrastructure instead
of reinventing the wheel.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>