linux: Handle process gone when detaching (#543)

This is an attempt to handle the situation where the process has gone
away and we're shutting down. I've observed the system looping
attempting to move the state out of ATTACHED. It's possible that the
state machine here needs to be made more complex (ALREADY_ATTACHED is
misleading).

Informs #478.

src/linux/frida-helper-backend.vala

@@ -1931,6 +1931,10 @@ namespace Frida {
 			try {
 				yield suspend (null);
 				close ();
+			} catch (Error e) {
+				// If the process is gone, then there's no point in retrying.
+				if (e is Error.PROCESS_NOT_FOUND)
+					attach_state = ALREADY_ATTACHED;
 			} catch (GLib.Error e) {
 			}
 		}