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/runtime/fenv.h

@@ -7,16 +7,24 @@
 #define FE_TOWARDZERO 0x0c00
 
 #define FE_INVALID    1
+#define __FE_DENORM   2
 #define FE_DIVBYZERO  4
 #define FE_OVERFLOW   8
 #define FE_UNDERFLOW  16
 #define FE_INEXACT    32
-#define FE_ALL_EXCEPT 61
+#define FE_ALL_EXCEPT 63
+
+#ifdef __FLT_EVAL_METHOD__
+#define FLT_EVAL_METHOD __FLT_EVAL_METHOD__
+#else
+#define FLT_EVAL_METHOD 0
+#endif
 
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 COSMOPOLITAN_C_START_
 
 #define FLT_ROUNDS (__flt_rounds())
+#define FE_DFL_ENV ((const fenv_t *)-1)
 
 typedef void *fenv_t;
 typedef uint16_t fexcept_t;