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

test/libc/runtime/sigsetjmp_test.c

@@ -0,0 +1,56 @@
+/*-*- 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/calls/calls.h"
+#include "libc/calls/struct/sigaction.h"
+#include "libc/calls/struct/siginfo.h"
+#include "libc/calls/struct/sigset.h"
+#include "libc/dce.h"
+#include "libc/runtime/runtime.h"
+#include "libc/sysv/consts/sa.h"
+#include "libc/sysv/consts/sig.h"
+#include "libc/testlib/testlib.h"
+
+sigjmp_buf jb;
+volatile int sigs;
+volatile int jumps;
+
+void OnSignal(int sig, siginfo_t *si, void *ctx) {
+  ++sigs;
+}
+
+TEST(sigsetjmp, test) {
+  if (IsWindows()) return;  // no sigusr1 support
+  sigset_t ss;
+  int i, n = 1000;
+  struct sigaction sa = {.sa_sigaction = OnSignal};
+  ASSERT_SYS(0, 0, sigaction(SIGUSR1, &sa, 0));
+  for (i = 0; i < n; ++i) {
+    if (!sigsetjmp(jb, 1)) {
+      sigemptyset(&ss);
+      sigaddset(&ss, SIGUSR1);
+      ASSERT_SYS(0, 0, sigprocmask(SIG_BLOCK, &ss, 0));
+      ASSERT_SYS(0, 0, raise(SIGUSR1));
+      siglongjmp(jb, 1);
+    } else {
+      ++jumps;
+    }
+  }
+  ASSERT_EQ(1000, jumps);
+  ASSERT_EQ(1000, sigs);
+}