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/dns/resolvednsreverse.c

@@ -34,6 +34,7 @@
 #include "libc/mem/mem.h"
 #include "libc/runtime/runtime.h"
 #include "libc/sock/sock.h"
+#include "libc/sock/struct/sockaddr.h"
 #include "libc/stdio/rand.h"
 #include "libc/str/str.h"
 #include "libc/sysv/consts/af.h"
@@ -75,7 +76,7 @@ int ResolveDnsReverse(const struct ResolvConf *resolvconf, int af,
   SerializeDnsHeader(msg, &h);
   if ((n = SerializeDnsQuestion(msg + 12, 500, &q)) == -1) return -1;
   if ((fd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) == -1) return -1;
-  if (sendto(fd, msg, 12 + n, 0, resolvconf->nameservers.p,
+  if (sendto(fd, msg, 12 + n, 0, (struct sockaddr *)resolvconf->nameservers.p,
              sizeof(*resolvconf->nameservers.p)) == 12 + n &&
       (n = read(fd, msg, 512)) >= 12) {
     DeserializeDnsHeader(&h2, msg);