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/intrin/describeflags.internal.h

@@ -60,6 +60,7 @@ const char *DescribeSocketProtocol(char[12], int);
 const char *DescribeSocketType(char[64], int);
 const char *DescribeStdioState(char[12], int);
 const char *DescribeWhence(char[12], int);
+const char *DescribeWhichPrio(char[12], int);
 
 #define DescribeArchPrctlCode(x)     DescribeArchPrctlCode(alloca(12), x)
 #define DescribeCapability(x)        DescribeCapability(alloca(20), x)
@@ -106,6 +107,7 @@ const char *DescribeWhence(char[12], int);
 #define DescribeSocketType(x)        DescribeSocketType(alloca(64), x)
 #define DescribeStdioState(x)        DescribeStdioState(alloca(12), x)
 #define DescribeWhence(x)            DescribeWhence(alloca(12), x)
+#define DescribeWhichPrio(x)         DescribeWhichPrio(alloca(12), x)
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */

libc/intrin/describewhichprio.c

@@ -0,0 +1,29 @@
+/*-*- 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/fmt/itoa.h"
+#include "libc/intrin/describeflags.internal.h"
+#include "libc/sysv/consts/prio.h"
+
+const char *(DescribeWhichPrio)(char buf[12], int x) {
+  if (x == PRIO_PROCESS) return "PRIO_PROCESS";
+  if (x == PRIO_PGRP) return "PRIO_PGRP";
+  if (x == PRIO_USER) return "PRIO_USER";
+  FormatInt32(buf, x);
+  return buf;
+}

libc/intrin/pthread_setspecific.c

@@ -23,7 +23,7 @@
 /**
  * Sets value of TLS slot for current thread.
  */
-int pthread_setspecific(pthread_key_t key, void *val) {
+int pthread_setspecific(pthread_key_t key, const void *val) {
   if (0 <= key && key < PTHREAD_KEYS_MAX) {
     __get_tls()->tib_keys[key] = val;
     return 0;