Make fixes and improvements

- Document more compiler flags
- Expose new __print_maps() api
- Better overflow checking in mmap()
- Improve the shell example somewhat
- Fix minor runtime bugs regarding stacks
- Make kill() on fork()+execve()'d children work
- Support CLONE_CHILD_CLEARTID for proper joining
- Fix recent possible deadlock regression with --ftrace

libc/str/strsignal.greg.c

@@ -35,7 +35,9 @@ static char g_strsignal[12];
  * @return pointer to static memory that mutates on subsequent calls
  * @see sigaction()
  */
-noasan noinstrument char *strsignal(int sig) {
+privileged char *strsignal(int sig) {
+  // we need privileged because:
+  //   kprintf is privileged and it depends on this
   char *p;
   const char *s;
   p = g_strsignal;