Actually set the 'got_verstring' flag in sshshare.c!

Actually set the 'got_verstring' flag in sshshare.c!

For each connection to a downstream I had a flag indicating that we'd
sent a version string to that downstream, and one indicating that we'd
received one in return. But I never actually set the latter to TRUE -
which was OK, as it turned out, because I never used it for anything
either.

Now I do want to use it, so I'd better actually set it :-)