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.

RemoteDaemon/Transport/Tor.hs

@@ -115,6 +115,7 @@ serveClient th@(TransportHandle _ _ rd) u r q = bracket setup cleanup start
 				, connCheckAuth = (`isAllowedAuthToken` allowed)
 				, connIhdl = P2PHandle h
 				, connOhdl = P2PHandle h
+				, connProcess = Nothing
 				, connIdent = ConnIdent $ Just "tor remotedaemon"
 				}
 			-- not really Client, but we don't know their uuid yet