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/calls/open.c

@@ -28,9 +28,10 @@
  * @param mode is an octal user/group/other permission signifier, that's
  *     ignored if O_CREAT or O_TMPFILE weren't passed
  * @return number needing close(), or -1 w/ errno
- * @asyncsignalsafe
+ * @asyncsignalsafe (zip files may have issues)
+ * @vforksafe (raises error if zip file)
  * @restartable
- * @vforksafe
+ * @threadsafe
  */
 int open(const char *file, int flags, ...) {
   va_list va;