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/sock/struct/msghdr.h

@@ -8,10 +8,11 @@ struct msghdr {            /* Linux+NT ABI */
   void *msg_name;          /* optional address */
   uint32_t msg_namelen;    /* size of msg_name */
   struct iovec *msg_iov;   /* scatter/gather array */
-  uint64_t msg_iovlen;     /* iovec count */
+  int msg_iovlen;          /* iovec count */
   void *msg_control;       /* credentials and stuff */
-  uint64_t msg_controllen; /* size of msg_control */
-  uint32_t msg_flags;      /* MSG_XXX */
+  uint32_t msg_controllen; /* size of msg_control */
+  uint32_t __pad0;         /* reconcile abi */
+  int msg_flags;           /* MSG_XXX */
 };
 
 ssize_t recvmsg(int, struct msghdr *, int);