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/unix_test.c

@@ -66,7 +66,7 @@ TEST(unix, datagram) {
   alarm(3);
   while (!fileexists(addr.sun_path)) usleep(10000);
   ASSERT_SYS(0, 3, socket(AF_UNIX, SOCK_DGRAM, 0));
-  ASSERT_SYS(0, 5, sendto(3, "hello", 5, 0, &addr, len));
+  ASSERT_SYS(0, 5, sendto(3, "hello", 5, 0, (struct sockaddr *)&addr, len));
   ASSERT_SYS(0, 0, close(3));
   ASSERT_NE(-1, wait(&ws));
   EXPECT_TRUE(WIFEXITED(ws));