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

test/libc/sock/setsockopt_test.c

@@ -38,7 +38,7 @@ TEST(setsockopt, SO_RCVTIMEO) {
   struct sockaddr_in sa = {AF_INET, 0, {htonl(0x7f000001)}};
   EXPECT_SYS(0, 3, socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP));
   EXPECT_SYS(0, 0, setsockopt(3, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv)));
-  EXPECT_SYS(0, 0, bind(3, &sa, sizeof(struct sockaddr_in)));
+  EXPECT_SYS(0, 0, bind(3, (struct sockaddr *)&sa, sizeof(struct sockaddr_in)));
   EXPECT_SYS(EAGAIN, -1, read(3, buf, sizeof(buf)));
   EXPECT_SYS(0, 0, close(3));
 }