add connProcess to P2PConnection

When using the new generic P2P transport to open an outgoing connection
to a peer, this will hold the pid of the git-annex-p2p-<netname>
command.

closeConnection simply waits for it. Rather than relying on garbage
collection of the closed handles to close it.

In Remote.Helper.Ssh, connProcess is set to Nothing, even though there
is a similar process being used there. That code stores the pid in
OpenConnection instead, and handles waiting for it itself. A bit ugly,
but not worth cleaning up at this point, maybe later.

Remote/Helper/Ssh.hs

@@ -270,6 +270,7 @@ openP2PShellConnection' r maxprotoversion bypass = do
 			, P2P.connCheckAuth = const False
 			, P2P.connIhdl = P2P.P2PHandle to
 			, P2P.connOhdl = P2P.P2PHandle from
+			, P2P.connProcess = Nothing
 			, P2P.connIdent = P2P.ConnIdent $
 				Just $ "git-annex-shell connection " ++ show pidnum
 			}