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/sock/sendto.c

@@ -18,14 +18,15 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
 #include "libc/calls/internal.h"
-#include "libc/intrin/strace.internal.h"
 #include "libc/calls/struct/iovec.h"
 #include "libc/calls/struct/iovec.internal.h"
 #include "libc/dce.h"
 #include "libc/intrin/asan.internal.h"
+#include "libc/intrin/strace.internal.h"
 #include "libc/macros.internal.h"
 #include "libc/sock/internal.h"
 #include "libc/sock/sock.h"
+#include "libc/sock/struct/sockaddr.h"
 #include "libc/sock/struct/sockaddr.internal.h"
 #include "libc/str/str.h"
 #include "libc/sysv/errfuns.h"
@@ -52,7 +53,7 @@
  * @restartable (unless SO_RCVTIMEO)
  */
 ssize_t sendto(int fd, const void *buf, size_t size, uint32_t flags,
-               const void *opt_addr, uint32_t addrsize) {
+               const struct sockaddr *opt_addr, uint32_t addrsize) {
   ssize_t rc;
   uint32_t bsdaddrsize;
   union sockaddr_storage_bsd bsd;