Improve signals and memory protection

- Document sigaction()
- Simplify New Technology fork() code
- Testing and many bug fixes for mprotect()
- Distribute Intel Xed ILD in the amalgamation
- Turn Xed enums into defines to avoid DWARF bloat
- Improve polyfilling of SA_SIGINFO on BSDs and fix bugs
- setpgid(getpid(), getpid()) on Windows will ignore CTRL-C
- Work around issues relating to NT mappings being executable
- Permit automatic executable stack override via `ape_stack_pf`

libc/calls/signal.c

@@ -28,7 +28,7 @@
  *
  * @return old signal handler on success or SIG_ERR w/ errno
  * @note this function has BSD semantics, i.e. SA_RESTART
- * @see sigaction() which has more features
+ * @see sigaction() which has more features and docs
  */
 sighandler_t(signal)(int sig, sighandler_t func) {
   struct sigaction old, sa = {.sa_handler = func, .sa_flags = SA_RESTART};