Fix: Improve handling of soft signals on darwin

Fixes a bug on OSX where, if the debugged process spawned a child, when
that process received a SIGCHLD it would cause Delve to hang.

Fixes #197

proc/proc_test.go

@@ -694,3 +694,13 @@ func TestBreakpointOnFunctionEntry(t *testing.T) {
 		}
 	})
 }
+
+func TestProcessReceivesSIGCHLD(t *testing.T) {
+	withTestProcess("sigchldprog", t, func(p *Process, fixture protest.Fixture) {
+		err := p.Continue()
+		_, ok := err.(ProcessExitedError)
+		if !ok {
+			t.Fatalf("Continue() returned unexpected error type %s", err)
+		}
+	})
+}