socks5 support

[Dan Kaminsky]

Whoa people, there are two forms of SOCKS support:

1) SOCKS to get to the SSH server
2) SOCKS to get to hosts behind the SSH server

In the former, the SSH client is a SOCKS client.  ProxyCommand does this.
In the latter, the SSH client is a SOCKS server.  ProxyCommand doesn't touch
this.

The latter is ridiculously useful (semi-VPN style behavior becomes possible,
especially on OSX with integrated OS level SOCKS).  The former is also
really useful for a decent set of places.

The way I see it, Dynamic Forwarding (SOCKS or HTTP to direct SSH tunnels)
is useful enough to justify its presence, and as long as we're going to have
the understanding of those protocols integrated into the SSH client anyway,
we might as well add the 5-20 lines of code to allow the SSH client to be a
SOCKS client too.  Save ProxyCommand for the really, really weird kind of
things that I like to do :-)

Markus -- send me your patches, I lost 'em (again *laughs*).

--Dan