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/thread/pthread_equal.c

@@ -17,7 +17,6 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/thread/thread.h"
-#include "libc/thread/posixthread.internal.h"
 
 /**
  * Compares thread ids;
@@ -25,7 +24,5 @@
  * @return nonzero if equal, otherwise zero
  */
 int pthread_equal(pthread_t t1, pthread_t t2) {
-  struct PosixThread *a = (struct PosixThread *)t1;
-  struct PosixThread *b = (struct PosixThread *)t2;
-  return a->tid == b->tid;
+  return t1 == t2;
 }