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/showcrashreports.c

@@ -111,7 +111,7 @@ void ShowCrashReports(void) {
   kCrashSigs[4] = SIGTRAP; /* bad system call */
   kCrashSigs[5] = SIGABRT; /* abort() called */
   kCrashSigs[6] = SIGBUS;  /* misaligned, noncanonical ptr, etc. */
-  kCrashSigs[7] = SIGSYS;  /* bad system call */
+  kCrashSigs[7] = SIGURG;  /* placeholder */
   /* </SYNC-LIST>: showcrashreports.c, oncrashthunks.S, oncrash.c */
   if (!IsWindows()) {
     bzero(&ss, sizeof(ss));