New command-line option in Plink (and PuTTY, though it's less useful

there): `plink host -nc host2:port' causes the SSH connection's main
channel to be replaced with a direct-tcpip connection to the
specified destination. This feature is mainly designed for use as a
local proxy: setting your local proxy command to `plink %proxyhost
-nc %host:%port' lets you tunnel SSH over SSH with a minimum of
fuss. Works on all platforms.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6823 cda61777-01e9-0310-a592-d414129be87e