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/sysv/consts/prio.h

@@ -9,7 +9,6 @@ extern const int PRIO_MIN;
 extern const int PRIO_PGRP;
 extern const int PRIO_PROCESS;
 extern const int PRIO_USER;
-extern const int NZERO;
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
@@ -19,6 +18,5 @@ COSMOPOLITAN_C_END_
 #define PRIO_USER    LITERALLY(2)
 #define PRIO_MIN     LITERALLY(-20)
 #define PRIO_MAX     LITERALLY(20)
-#define NZERO        LITERALLY(20)
 
 #endif /* COSMOPOLITAN_LIBC_SYSV_CONSTS_PRIO_H_ */