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/oncrashthunks.S

@@ -90,13 +90,13 @@ __oncrash_sigbus:
 	.endfn	__oncrash_sigbus,globl
 
 	.org	11*7
-__oncrash_sigsys:
+__oncrash_sigurg:
 	push	%rbp
 	mov	%rsp,%rbp
 	call	__oncrash
 	pop	%rbp
 	ret
-	.endfn	__oncrash_sigsys,globl
+	.endfn	__oncrash_sigurg,globl
 
 //	</SYNC-LIST>: showcrashreports.c, oncrashthunks.S, oncrash.c, internal.h