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/intrin/promises.c

@@ -19,5 +19,6 @@
 #include "libc/intrin/promises.internal.h"
 
 // XXX: should be inherited thread local
+unsigned __pledge_mode;
 unsigned long __promises;
 unsigned long __execpromises;