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/posix_fadvise.c

@@ -44,7 +44,7 @@ int sys_fadvise_netbsd(int, int, int64_t, int64_t, int) asm("sys_fadvise");
  * @returnserrno
  * @threadsafe
  */
-errno_t posix_fadvise(int fd, uint64_t offset, uint64_t len, int advice) {
+errno_t posix_fadvise(int fd, int64_t offset, int64_t len, int advice) {
   int rc, e = errno;
   if (IsLinux()) {
     rc = sys_fadvise(fd, offset, len, advice);