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/tinymath/powf.c

@@ -37,7 +37,7 @@ asm(".ident\t\"\\n\\n\
 Double-precision math functions (MIT License)\\n\
 Copyright 2018 ARM Limited\"");
 asm(".include \"libc/disclaimer.inc\"");
-/* clang-format off */
+// clang-format off
 
 /*
  * Copyright (c) 2017-2018, Arm Limited.