Bug 904

A lot of people use SSH for DMZs, and thus you have to ssh through several hosts
(4 hops through one environment I'm familiar with!)  It would be nifty to see
syntax something like:

ssh user1@hop1/hop2/hop3/user4@hop4 reboot

This would ssh to hop1 as user1, then to hop2 as user1, then to hop3 as user1
and then hop4 as user4 and execute the reboot command.  The '/' could be
replaced with another character, doesn't matter, so long as there is some way to
represent this.

The same syntax should work for scp and sftp as well.

As part of this it'd really be helpful to have port forwarding integrated, so I
could do:

ssh user1@hop1/hop2/hop3/user4@hop4 -L 500 -R 5000 sleep 3600

Which would do the same as above except execute the sleep 3600 command, and
forward the local port 500 (on the host you are on) to remote port 5000 on hop4.

This would fix a major hassle because right now doing something like that
requires naming ports in the intermediate hops, and there isn't any way to say
"pick a random port, I don't care, I just want to connect both ends".  As a
result everyone port forwarding through like this needs to use a different port
number, and they have to wait for the ports to time out if they need to reconnect.