Centralize libpq's low-level code for dropping a connection.

Create an internal function pqDropConnection that does the physical socket
close and cleans up closely-associated state.  This removes a bunch of ad
hoc, not always consistent closure code.  The ulterior motive is to have a
single place to wait for a spawned child backend to exit, but this seems
like good cleanup even if that never happens.

I went back and forth on whether to include "conn->status = CONNECTION_BAD"
in pqDropConnection's actions, but for the moment decided not to.  Only a
minority of the call sites actually want that, and in any case it's
arguable that conn->status is slightly higher-level state, and thus not
part of this function's purview.

src/interfaces/libpq/fe-misc.c

@@ -780,11 +780,8 @@ retry4:
 	 * has been set already.
 	 */
 definitelyFailed:
+	pqDropConnection(conn);
 	conn->status = CONNECTION_BAD;		/* No more connection to backend */
-	pqsecure_close(conn);
-	closesocket(conn->sock);
-	conn->sock = -1;
-
 	return -1;
 }