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/math.h

@@ -64,8 +64,8 @@
 #define FP_ZERO      2
 #define FP_SUBNORMAL 3
 #define FP_NORMAL    4
-#define FP_ILOGB0    (-2147483647-1)
-#define FP_ILOGBNAN  (-2147483647-1)
+#define FP_ILOGB0    (-2147483647 - 1)
+#define FP_ILOGBNAN  (-2147483647 - 1)
 
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 COSMOPOLITAN_C_START_
@@ -99,6 +99,8 @@ typedef double double_t;
 #define fpclassify(x) \
   __builtin_fpclassify(FP_NAN, FP_INFINITE, FP_NORMAL, FP_SUBNORMAL, FP_ZERO, x)
 
+extern int signgam;
+
 double acos(double);
 double acosh(double);
 double asin(double);
@@ -307,6 +309,20 @@ void sincosl(long double, long double *, long double *);
 float fsumf(const float *, size_t);
 double fsum(const double *, size_t);
 
+double j0(double);
+double j1(double);
+double jn(int, double);
+float j0f(float);
+float j1f(float);
+float jnf(int, float);
+
+double y0(double);
+double y1(double);
+double yn(int, double);
+float y0f(float);
+float y1f(float);
+float ynf(int, float);
+
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */
 #endif /* COSMOPOLITAN_LIBC_MATH_H_ */