Add torture test for zipos file descriptors

This change hardens the code for opening /zip/ files using the system
call interface. Thread safety and signal safety has been improved for
file descriptors in general. We now document fixed addresses that are
needed for low level allocations.

libc/intrin/tls.greg.c

@@ -61,18 +61,30 @@ privileged void *__initialize_tls(char tib[64]) {
  * Installs thread information block on main process.
  *
  * For example, to set up TLS correctly for the main thread, without
- * creating any threads using `clone` (which does this automatically),
- * it is sufficient to say:
+ * creating any threads, then it's sufficient to say:
  *
  *     __attribute__((__constructor__)) static void InitTls(void) {
  *       static char tls[64];
  *       __initialize_tls(tls);
- *       __threaded = *(int *)(tls + 0x38) = gettid();
+ *       *(int *)(tls + 0x38) = gettid();
  *       *(int *)(tls + 0x3c) = __errno;
  *       __install_tls(tls);
  *     }
  *
- * Since that'll ensure it happens exactly once.
+ * We use a constructor here to make sure it only happens once. Please
+ * note that calling `clone` will do this automatically.
+ *
+ * Installing TLS causes the `__tls_enabled` variable to be set. This
+ * causes C library features such as `errno` and `gettid()` to use TLS.
+ * This can help things like recursive mutexes go significantly faster.
+ *
+ * To access your TLS storage, you can call `__get_tls()` or
+ * __get_tls_inline()` which return the address of the `tib`.
+ *
+ * @param tib is your thread information block, which must have at least
+ *     64 bytes on the righthand side of the tib pointer since those are
+ *     the values your C library reserves for itself. memory on the left
+ *     side of the pointer is reserved by the linker for _Thread_local.
  */
 privileged void __install_tls(char tib[64]) {
   int ax, dx;