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/freeaddrinfo.c

@@ -23,7 +23,7 @@
  * Frees addresses returned by getaddrinfo().
  * @threadsafe
  */
-int freeaddrinfo(struct addrinfo *ai) {
+void freeaddrinfo(struct addrinfo *ai) {
   struct addrinfo *next;
   while (ai) {
     /* we assume ai_addr and ai_canonname are shoehorned */
@@ -31,5 +31,4 @@ int freeaddrinfo(struct addrinfo *ai) {
     free(ai);
     ai = next;
   }
-  return 0;
 }