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/log/backtrace.c

@@ -113,13 +113,12 @@ int main(int argc, char *argv[]) {
         pDataOverrunCrash(10 + 1);
         exit(0);
       case 5:
-        exit((intptr_t)pStackOverrunCrash(10 + 1));
+        exit((intptr_t)pStackOverrunCrash(10 + 10000));
       case 6:
         exit((intptr_t)pMemoryLeakCrash());
       case 7:
         exit(pNpeCrash(0));
       case 8:
-        __cxa_finalize(0);
         exit(pNpeCrash(0));
       case 9:
         exit(pStackOverflow(pStackOverflow, 0));