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/calls/tkill.c

@@ -48,6 +48,10 @@ static textwindows int sys_tkill_nt(int tid, int sig) {
  * @param tid is thread id
  * @param sig does nothing on xnu
  * @return 0 on success, or -1 w/ errno
+ * @raise ESRCH if `tid` was valid but no such thread existed
+ * @raise EAGAIN if `RLIMIT_SIGPENDING` was exceeded
+ * @raise EINVAL if `tid` or `sig` was invalid
+ * @raise EPERM if permission was denied
  * @asyncsignalsafe
  */
 int tkill(int tid, int sig) {