Separate out the code for creating and sending SSH version strings so that in

Separate out the code for creating and sending SSH version strings so that in
the SSH-2-only case, we can send it as soon as we connect rather than waiting
for the server's one.  Unfortunately, actually doing so will take a little
more effort -- there are subtleties to do with having a working log context
at the right moment that need to be sorted out.

git-svn-id: http://svn.tartarus.org/sgt/putty@7645 cda61777-01e9-0310-a592-d414129be87e