Improve cosmo's conformance to libc-test

This change addresses various open source compatibility issues, so that
we pass 313/411 of the tests in https://github.com/jart/libc-test where
earlier today we were passing about 30/411 of them, due to header toil.
Please note that Glibc only passes 341/411 so 313 today is pretty good!

- Make the conformance of libc/isystem/ headers nearly perfect
- Import more of the remaining math library routines from Musl
- Fix inconsistencies with type signatures of calls like umask
- Write tests for getpriority/setpriority which work great now
- conform to `struct sockaddr *` on remaining socket functions
- Import a bunch of uninteresting stdlib functions e.g. rand48
- Introduce readdir_r, scandir, pthread_kill, sigsetjmp, etc..

Follow the instructions in our `tool/scripts/cosmocc` toolchain to run
these tests yourself. You use `make CC=cosmocc` on the test repository

libc/thread/pthread_cancel.c

@@ -34,7 +34,7 @@
  *     void OnUsr1(int sig) { gotusr1 = true; }
  *     struct sigaction sa = {.sa_handler = OnUsr1};
  *     sigaction(SIGUSR1, &sa, 0);
- *     tkill(pthread_getunique_np(thread), SIGUSR1);
+ *     pthread_kill(thread, SIGUSR1);
  *
  * The above code should successfully cancel a thread's blocking io
  * operations in most cases, e.g.
@@ -61,7 +61,7 @@
  * @see https://sourceware.org/bugzilla/show_bug.cgi?id=12683
  */
 int pthread_cancel(pthread_t thread) {
-  kprintf("error: pthread_cancel() is unsupported, please read the "
-          "cosmopolitan libc documentation for further details\n");
+  kprintf("error: pthread_cancel() isn't supported, please see the"
+          " cosmopolitan libc documentation for further details\n");
   _Exit(1);
 }