Implement connection sharing between instances of PuTTY.

Implement connection sharing between instances of PuTTY.

The basic strategy is described at the top of the new source file
sshshare.c. In very brief: an 'upstream' PuTTY opens a Unix-domain
socket or Windows named pipe, and listens for connections from other
PuTTYs wanting to run sessions on the same server. The protocol spoken
down that socket/pipe is essentially the bare ssh-connection protocol,
using a trivial binary packet protocol with no encryption, and the
upstream has to do some fiddly transformations that I've been
referring to as 'channel-number NAT' to avoid resource clashes between
the sessions it's managing.

This is quite different from OpenSSH's approach of using the Unix-
domain socket as a means of passing file descriptors around; the main
reason for that is that fd-passing is Unix-specific but this system
has to work on Windows too. However, there are additional advantages,
such as making it easy for each downstream PuTTY to run its own
independent set of port and X11 forwardings (though the method for
making the latter work is quite painful).

Sharing is off by default, but configuration is intended to be very
easy in the normal case - just tick one box in the SSH config panel
and everything else happens automatically.

[originally from svn r10083]