Merge branch 't/sb'

Was cooking in next for several weeks with no problems so far.

	fd864c Fix documentation of pass_buffer_as_shm().
	32ffc0 Pass command exit status via sideband to client.
	7dc1b5 Send command output via sideband.
	0229d7 Introduce send_strerror().
	d5a9e8 Simplify pass_buffer_as_shm().
	3667d7 command.c: Improve get_status().
	aa3fe7 Pass command via sideband.
	9c2a26 Use sideband also for "proceed" handshake.
	4744d9 Use sideband also for challenge response.
	fa9e0e Send and receive challenge via sideband.
	2830b9 Add sideband implementation.
	e151db Implement client-server feature negotiation.
	e31d5d afs: Rename sc_send_result() to afs_cb_result_handler().
	68cb0a Introduce afs_max_size_handler_data and afs_max_size_handler().
	d04b83 client: Improve error diagnostics.

diff --cc NEWS
index 0ac2240effe91e4294c8ec901fe18f9c143e42d3,0ac2240effe91e4294c8ec901fe18f9c143e42d3..fa8e0249be9945f08cd4bf9c36531854cf8000f5
--- 1/NEWS
--- 2/NEWS
+++ b/NEWS
@@@ -2,6 -2,6 +2,8 @@@
  0.4.11 (to be announced) "mutual diversity"
  -------------------------------------------
  
++      - Sideband connections: If both para_server and para_client
++        support this feature, data is sent as a multiplexed stream.
        - The --no_default_filters option of para_filter has been
          removed.
        - Documentation improvements.