Gave asynchronous operations approximate POSIX signal semantics. This

will enable us to cascade async operations, which is necessary in order to
properly support DNS.  (For example, an HTTP request may have to redirect
to a new location and will have to perform a new DNS lookup, so we can't
just rely on doing the name lookup at the time of parsing the initial
URL).

Anything other than HTTP is probably broken right now; I'll fix the others
up asap.

src/include/gpxe/dhcp.h

@@ -466,7 +466,7 @@ struct dhcp_session {
 	 */
 	int state;
 	/** Asynchronous operation for this DHCP session */
-	struct async_operation aop;
+	struct async async;
 	/** Retransmission timer */
 	struct retry_timer timer;
 };
@@ -504,6 +504,6 @@ extern int create_dhcp_packet ( struct net_device *netdev, uint8_t msgtype,
 				struct dhcp_packet *dhcppkt );
 extern int copy_dhcp_packet_options ( struct dhcp_packet *dhcppkt,
 				      struct dhcp_option_block *options );
-extern struct async_operation * start_dhcp ( struct dhcp_session *dhcp );
+extern int start_dhcp ( struct dhcp_session *dhcp, struct async *parent );
 
 #endif /* _GPXE_DHCP_H */