Add DemuxConn and `bup mux` for client-server

Add DemuxConn and `bup mux` for client-server

`bup mux` works with any bup command to multiplex its stdout and stderr
streams over a single stdout stream.

DemuxConn works on the client side to demultiplex stderr and data
streams from a single stream, emulating a simple connection.

For now, these are only used in the case of simple socket bup://
client-server connections, because rsh and local connections don't need
them.

Signed-off-by: Brandon Low <lostlogic@lostlogicx.com>