feat: add selfstoptarget and sigchldchild

user/sigdefault.c

@@ -0,0 +1,26 @@
+#include "ulib.h"
+
+/*
+ * Only ever spawned by sigtest.c. Installs no handler at all, so the
+ * sigTerm sigtest.c sends it must run the OTHER branch of
+ * signal.c's sigConsumePending: default disposition, which
+ * isr.c's deliverPendingSignals turns into a real threadExit -- proving a
+ * signal with no handler actually terminates its target (exit code
+ * 128+signum, see isr.c), not just that a handler-based one can be caught.
+ * Reaching the FAIL print below would mean the default-terminate path
+ * never fired at all.
+ */
+
+int main(void)
+{
+    print("sigdefault: start, pid=");
+    printDec(userGetpid());
+    print("\n");
+    print("sigdefault: waiting to be terminated by default disposition\n");
+
+    for (int i = 0; i < 40; i++)
+        userSleep(20);
+
+    print("sigdefault: FAIL still alive after default-disposition signal\n");
+    return 1;
+}