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

libc/calls/prctl.c

@@ -26,7 +26,7 @@
 /**
  * Tunes process on Linux.
  *
- * @raise ENOSYS on non-Linux.
+ * @raise ENOSYS on non-Linux
  */
 privileged int prctl(int operation, ...) {
   int rc;