Improve system calls

- Wrap clock_getres()
- Wrap sched_setscheduler()
- Make sleep() api conformant
- Polyfill sleep() using select()
- Improve clock_gettime() polyfill
- Make nanosleep() POSIX conformant
- Slightly improve some DNS functions
- Further strengthen pledge() sandboxing
- Improve rounding of timeval / timespec
- Allow layering of pledge() calls on Linux
- Polyfill sched_yield() using select() on XNU
- Delete more system constants we probably don't need

examples/hello4.c

@@ -13,6 +13,6 @@
 int main(int argc, char *argv[]) {
   volatile long double x = -.5;
   volatile long double y = 1.5;
-  printf("atan2l(%.19Lg, %.19Lg) is %.19Lg\n", x, y, atan2l(x, y));
+  printf("Hello World! %.19Lg\n", atan2l(x, y));
   return 0;
 }