Fix bugs and make code tinier

- Fixed bug where stdio eof wasn't being sticky
- Fixed bug where fseeko() wasn't clearing eof state
- Removed assert() usage from libc favoring _unassert() / _npassert()

libc/intrin/isatleastwindows10.greg.c

@@ -26,6 +26,6 @@
  * This function may only be called if IsWindows() is true.
  */
 privileged bool(IsAtLeastWindows10)(void) {
-  assert(IsWindows());
+  _unassert(IsWindows());
   return IsAtLeastWindows10();
 }