feat: add eintrtarget and envprint

user/exectarget.c

@@ -0,0 +1,19 @@
+#include "ulib.h"
+
+/*
+ * Never spawned directly by kmain.c or by proctest.c's userSpawn -- the
+ * only way this binary's "exectarget: running" line can ever appear in the
+ * boot log is if proctest.c's userExec("/bin/exectarget") call actually
+ * replaced proctest's own image with this one (see proctest.c). Its pid is
+ * therefore the SAME pid proctest was given at spawnUser time in kmain.c,
+ * which is what kmain.c's execReaperThread checks via threadJoin to prove
+ * exec() preserves process identity, not just that some program ran.
+ */
+
+int main(void)
+{
+    print("exectarget: running, pid=");
+    printDec(userGetpid());
+    print("\n");
+    return 7;
+}