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/dlmalloc/dlmalloc.c

@@ -380,9 +380,14 @@ int dlmalloc_sys_trim(struct MallocState *m, size_t pad) {
             size_t newsize = sp->size - extra;
             (void)newsize; /* placate people compiling -Wunused-variable */
             /* Prefer mremap, fall back to munmap */
-            if ((mremap(sp->base, sp->size, newsize, 0, 0) != MAP_FAILED) ||
-                (munmap(sp->base + newsize, extra) == 0)) {
+            int err = errno;
+            if (mremap(sp->base, sp->size, newsize, 0, 0) != MAP_FAILED) {
               released = extra;
+            } else {
+              errno = err;
+              if (!munmap(sp->base + newsize, extra)) {
+                released = extra;
+              }
             }
           }
         }