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`

examples/ctrlc.c

@@ -36,7 +36,7 @@ int main(int argc, char *argv[]) {
   fprintf(stderr, "This echos stdio until Ctrl+C is pressed.\n");
   CHECK_NE(-1, sigaction(SIGINT, &saint, NULL));
   for (;;) {
-    rc = read(0, buf, BUFSIZ);
+    rc = read(0, buf, 512);
     if (rc != -1) {
       got = rc;
     } else {