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/struct/siginfo.h

@@ -15,7 +15,7 @@ struct siginfo {
           uint32_t si_uid;
         };
         struct {
-          int32_t si_timerid;
+          int32_t si_timerid; /* SIGALRM */
           int32_t si_overrun;
         };
       };
@@ -23,7 +23,8 @@ struct siginfo {
         union sigval si_value; /* provided by third arg of sigqueue(2) */
         struct {
           int32_t si_status;
-          int64_t si_utime, si_stime;
+          int64_t si_utime;
+          int64_t si_stime;
         };
       };
     };
@@ -39,7 +40,7 @@ struct siginfo {
       };
     };
     struct {
-      int64_t si_band;
+      int64_t si_band; /* SIGPOLL */
       int32_t si_fd;
     };
     struct {