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/intrin/describeflags.internal.h

@@ -60,6 +60,7 @@ const char *DescribeSocketProtocol(char[12], int);
 const char *DescribeSocketType(char[64], int);
 const char *DescribeStdioState(char[12], int);
 const char *DescribeWhence(char[12], int);
+const char *DescribeWhichPrio(char[12], int);
 
 #define DescribeArchPrctlCode(x)     DescribeArchPrctlCode(alloca(12), x)
 #define DescribeCapability(x)        DescribeCapability(alloca(20), x)
@@ -106,6 +107,7 @@ const char *DescribeWhence(char[12], int);
 #define DescribeSocketType(x)        DescribeSocketType(alloca(64), x)
 #define DescribeStdioState(x)        DescribeStdioState(alloca(12), x)
 #define DescribeWhence(x)            DescribeWhence(alloca(12), x)
+#define DescribeWhichPrio(x)         DescribeWhichPrio(alloca(12), x)
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */