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/sysv/consts/loglevel.h

@@ -0,0 +1,19 @@
+#ifndef COSMOPOLITAN_LIBC_SYSV_CONSTS_LOGLEVEL_H_
+#define COSMOPOLITAN_LIBC_SYSV_CONSTS_LOGLEVEL_H_
+#if !(__ASSEMBLER__ + __LINKER__ + 0)
+COSMOPOLITAN_C_START_
+
+#define LOGLEVEL_SCHED   -2
+#define LOGLEVEL_DEFAULT -1
+#define LOGLEVEL_EMERG   0
+#define LOGLEVEL_ALERT   1
+#define LOGLEVEL_CRIT    2
+#define LOGLEVEL_ERR     3
+#define LOGLEVEL_WARNING 4
+#define LOGLEVEL_NOTICE  5
+#define LOGLEVEL_INFO    6
+#define LOGLEVEL_DEBUG   7
+
+COSMOPOLITAN_C_END_
+#endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
+#endif /* COSMOPOLITAN_LIBC_SYSV_CONSTS_LOGLEVEL_H_ */