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_setcancelstate.c

@@ -0,0 +1,47 @@
+/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
+│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
+╞══════════════════════════════════════════════════════════════════════════════╡
+│ Copyright 2022 Justine Alexandra Roberts Tunney                              │
+│                                                                              │
+│ Permission to use, copy, modify, and/or distribute this software for         │
+│ any purpose with or without fee is hereby granted, provided that the         │
+│ above copyright notice and this permission notice appear in all copies.      │
+│                                                                              │
+│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
+│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
+│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
+│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
+│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
+│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
+│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
+│ PERFORMANCE OF THIS SOFTWARE.                                                │
+╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/errno.h"
+#include "libc/thread/posixthread.internal.h"
+#include "libc/thread/thread.h"
+#include "libc/thread/tls.h"
+
+/**
+ * Sets cancelability state.
+ *
+ * @param state may be one of:
+ *     - `PTHREAD_CANCEL_ENABLE` (default)
+ *     - `PTHREAD_CANCEL_DISABLE`
+ * @param oldstate optionally receives old value
+ * @return 0 on success, or errno on error
+ * @raise EINVAL if `state` has bad value
+ * @see pthread_cancel() for docs
+ */
+int pthread_setcancelstate(int state, int *oldstate) {
+  struct PosixThread *pt;
+  switch (state) {
+    case PTHREAD_CANCEL_ENABLE:
+    case PTHREAD_CANCEL_DISABLE:
+      pt = (struct PosixThread *)__get_tls()->tib_pthread;
+      if (oldstate) *oldstate = pt->canceldisable;
+      pt->canceldisable = state;
+      return 0;
+    default:
+      return EINVAL;
+  }
+}