Improve pledge() usability and consistency

- We now kill the program on violations like OpenBSD
- We now print a message explaining which promise is needed
- This change also fixes a linkage bug with thread local storage
- Your sigaction() handlers should now be more thread safe

A new `__pledge_mode` global has been introduced to make pledge() more
customizable on Linux. For example:

    __attribute__((__constructor__)) static void init(void) {
      __pledge_mode = SECCOMP_RET_ERRNO | EPERM;
    }

Can be used to restore our old permissive pledge() behavior.

libc/log/getsicodename.c

@@ -158,6 +158,11 @@ const char *GetSiCodeName(int sig, int si_code) {
     } else if (si_code == POLL_HUP) {
       strcpy(b + 5, "HUP"); /* device disconnected */
     }
+  } else if (sig == SIGSYS) {
+    NameIt(b, "SYS_", si_code);
+    if (si_code == SYS_SECCOMP) {
+      strcpy(b + 4, "SECCOMP");
+    }
   }
   return b;
 }