Support non-blocking i/o across platforms

This change introduces new tests for `O_NONBLOCK` and `SOCK_NONBLOCK` to
confirm that non-blocking i/o is now working on all supported platforms,
including Windows. For example, you can now say on Windows, MacOS, etc.:

    socket(AF_INET, SOCK_STREAM | SOCK_NONBLOCK, IPPROTO_TCP);

To create a non-blocking IPv4 TCP socket. Or you can enable non-blocking
i/o on an existing socket / pipe / etc. file descriptor by calling fcntl

    fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) | O_NONBLOCK);

This functionality is polyfilled on older Linux kernels too, e.g. RHEL5.
Now that fcntl() support is much better the FIOCLEX / FIONCLEX polyfills
for ioctl() have been removed since they're ugly non-POSIX diameond APIs

This change fixes a weakness in kprintf() that was causing Windows trace
tools to frequently crash.

libc/thread/pthread_attr_init.c

@@ -23,6 +23,15 @@
  * Initializes pthread attributes.
  *
  * @return 0 on success, or errno on error
+ * @see pthread_attr_setdetachstate()
+ * @see pthread_attr_setsigmask_np()
+ * @see pthread_attr_setstack()
+ * @see pthread_attr_setstacksize()
+ * @see pthread_attr_setguardsize()
+ * @see pthread_attr_setschedparam()
+ * @see pthread_attr_setschedpolicy()
+ * @see pthread_attr_setinheritsched()
+ * @see pthread_attr_setscope()
  */
 errno_t pthread_attr_init(pthread_attr_t *attr) {
   *attr = (pthread_attr_t){