Perform some code maintenance

- Change IDT code so kprintf() isn't mandatory dependency
- Document current intentions around pthread_cancel()
- Make _npassert() an _unassert() in MODE=tiny

libc/thread/pthread_cancel.c

@@ -16,9 +16,52 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
-#include "libc/errno.h"
+#include "libc/intrin/kprintf.h"
+#include "libc/runtime/runtime.h"
 #include "libc/thread/thread.h"
 
+/**
+ * Cancels thread.
+ *
+ * This function currently isn't supported. In order to support this
+ * function we'd need to redesign the system call interface, and add
+ * bloat and complexity to every function that can return EINTR. You
+ * might want to consider using `nsync_note` instead, which provides
+ * much better cancellations because posix cancellations is a broken
+ * design. If you need to cancel i/o operations, try doing this:
+ *
+ *     _Thread_local bool gotusr1;
+ *     void OnUsr1(int sig) { gotusr1 = true; }
+ *     struct sigaction sa = {.sa_handler = OnUsr1};
+ *     sigaction(SIGUSR1, &sa, 0);
+ *     tkill(pthread_getunique_np(thread), SIGUSR1);
+ *
+ * The above code should successfully cancel a thread's blocking io
+ * operations in most cases, e.g.
+ *
+ *     void *MyThread(void *arg) {
+ *       sigset_t ss;
+ *       sigfillset(&ss);
+ *       sigdelset(&ss, SIGUSR1);
+ *       sigprocmask(SIG_SETMASK, &ss, 0);
+ *       while (!gotusr1) {
+ *         char buf[512];
+ *         ssize_t rc = read(0, buf, sizeof(buf));
+ *         if (rc == -1 && errno == EINTR) continue;
+ *         write(1, buf, rc);
+ *       }
+ *       return 0;
+ *     }
+ *
+ * This has the same correctness issue as glibc, but it's usually
+ * "good enough" if you only need cancellations to perform things
+ * like server shutdown and socket options like `SO_RCVTIMEO` can
+ * ensure it's even safer, since it can't possibly block forever.
+ *
+ * @see https://sourceware.org/bugzilla/show_bug.cgi?id=12683
+ */
 int pthread_cancel(pthread_t thread) {
-  return ESRCH;
+  kprintf("error: pthread_cancel() is unsupported, please read the "
+          "cosmopolitan libc documentation for further details\n");
+  _Exit(1);
 }