Refactor signing code to remove most dependencies on 'struct cli'.

Refactor signing code to remove most dependencies on 'struct cli'.
Ensure a server can't do a downgrade attack if client signing is mandatory.
Add a lp_server_signing() function and a 'server signing' parameter that
will act as the client one does.
Jeremy
(This used to be commit 203e4bf0bfb66fd9239e9a0656438a71280113cb)