Add a ProxyJump ssh_config(5) option and corresponding -J ssh(1)

Add a ProxyJump ssh_config(5) option and corresponding -J ssh(1)
command-line flag to allow simplified indirection through a
SSH bastion or "jump host".

These options construct a proxy command that connects to the
specified jump host(s) (more than one may be specified) and uses
port-forwarding to establish a connection to the next destination.

This codifies the safest way of indirecting connections through SSH
servers and makes it easy to use.

ok markus@