upstream commit

We accidentally send an empty string and a zero uint32 with
 every direct-streamlocal@openssh.com channel open, in contravention of our
 own spec.

Fixing this is too hard wrt existing versions that expect these
fields to be present and fatal() if they aren't, so document them
as "reserved" fields in the PROTOCOL spec as though we always
intended this and let us never speak of it again.

bz#2529, reported by Ron Frederick

Upstream-ID: 34cd326a4d236ca6e39084c4ff796bd97ab833e7
