When starting a SOCKS connection in dynamic forwarding, freeze the

When starting a SOCKS connection in dynamic forwarding, freeze the
local socket _before_ calling the SSH setup functions. This makes no
difference to ssh.c itself, but it makes portfwd.c easier to reuse
for other purposes (e.g. as a component of a standalone SOCKS
server), because now ssh_send_port_open() can itself call
pfd_confirm() without the freeze and unfreeze happening in the wrong
order.

[originally from svn r8500]