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.

Command/EnableTor.hs

@@ -138,6 +138,7 @@ checkHiddenService = bracket setup cleanup go
 			, connCheckAuth = const False
 			, connIhdl = P2PHandle h
 			, connOhdl = P2PHandle h
+			, connProcess = Nothing
 			, connIdent = ConnIdent Nothing
 			}
 		runst <- mkRunState Client