feat: add selfstoptarget and sigchldchild

user/sigchldchild.c

@@ -0,0 +1,24 @@
+#include "ulib.h"
+
+/*
+ * Only ever spawned by sigchldtest.c. Does nothing rendezvous-shaped
+ * (unlike eintrtarget.c/masktarget.c's own targets) because it does not
+ * need to: sigchldtest.c's own proof (a real sigChld raised by the kernel
+ * against whoever spawned this process, sched.c's parentId/threadExit)
+ * does not depend on any particular ordering between this process's own
+ * death and sigchldtest.c noticing it -- sigRaise sets the pending bit on
+ * the parent unconditionally, whatever its state is at that instant, and
+ * sigchldtest.c's own bounded poll loop (the same "check a flag every
+ * userSleep(20)" idiom masktarget.c/nesttarget.c already use) picks it up
+ * on its very next natural wake either way. Just runs and exits with a
+ * distinctive code sigchldtest.c can verify through userWait after the
+ * fact.
+ */
+int main(void)
+{
+    print("sigchldchild: start, pid=");
+    printDec(userGetpid());
+    print("\n");
+    print("sigchldchild: exiting now\n");
+    return 84;
+}