[obexd] Disconnect request at the end of client session. Fixes JB#9622.

PTS test TC_CLIENT_OPH_BV-34-I requires that OBEX push client sends a disconnect request to the server, although IrOBEX standard allows the client to disconnect simply by closing the transport. 

Patch the push client to send a disconnect request and wait for a reply before closing the transport. If creating a disconnection request fails, just drop the transport; ignore whatever response headers are given by the server, they are not important as we're going away anyway. 

NB. Nokia N9 does send a disconnection request at the end of a client session: looks like this feature got lost somewhere between obexd 0.42 and 0.48.