sshd: support the ProxyJump ssh config

sshd: support the ProxyJump ssh config

This is useful to access git repositories behind a bastion server
(jump host).

Add a constant for the config; rewrite the whole connection initiation
to parse the value and (recursively) set up the chain of hops. Add
tests for a single hop and two different ways to configure a two-hop
chain.

The connection timeout applies to each hop in the chain individually.

Change-Id: Idd25af95aa2ec5367404587e4e530b0663c03665
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>