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`

third_party/xed/x86ild.greg.c

@@ -1115,7 +1115,7 @@ privileged static void xed_decode_instruction_length(
  * Clears instruction decoder state.
  */
 privileged struct XedDecodedInst *xed_decoded_inst_zero_set_mode(
-    struct XedDecodedInst *p, enum XedMachineMode mmode) {
+    struct XedDecodedInst *p, int mmode) {
   __builtin_memset(p, 0, sizeof(*p));
   xed_operands_set_mode(&p->op, mmode);
   return p;
@@ -1131,8 +1131,8 @@ privileged struct XedDecodedInst *xed_decoded_inst_zero_set_mode(
  * @note binary footprint increases ~4kb if this is used
  * @see biggest code in gdb/clang/tensorflow binaries
  */
-privileged enum XedError xed_instruction_length_decode(
-    struct XedDecodedInst *xedd, const void *itext, size_t bytes) {
+privileged int xed_instruction_length_decode(struct XedDecodedInst *xedd,
+                                             const void *itext, size_t bytes) {
   __builtin_memcpy(xedd->bytes, itext, MIN(15, bytes));
   xedd->op.max_bytes = MIN(15, bytes);
   xed_decode_instruction_length(xedd);