Use sideband also for "proceed" handshake.

Use sideband also for "proceed" handshake.

This adds sideband-aware code for the next step of the connection
handshake. If sideband connections are supported at both ends,
the server side sends an empty sideband packet with designator
SBD_PROCEED if the client was authenticated successfully.  When the
client receives this packet, it enters the CL_RECEIVED_PROCEED state,
just as with non-sideband connections.