Refactor the construction of X protocol greetings.

Refactor the construction of X protocol greetings.

I've moved it out into a separate function, preparatory to calling it
from somewhere completely different in changes to come. Also, we now
retain the peer address sent from the SSH server in string form,
rather than translating it immediately into a numeric IP address, so
that its original form will be available later to pass on elsewhere.

[originally from svn r10080]