Don't show HUD reflection on water surface

For reflectclip > 1 player's HUD reflects on water surface.
It fixes https://github.com/assaultcube/AC/issues/75
Optimize and fix autodownload feature

The commit:
1. fixes value of download speed in KB;
2. introduces unique names of progress_callback() functions;
3. removes one unnecessary write_callback() function;
4. removes duplicated code/info in loadmodel().
track missing models without "nomodel"-crutch

* missing models now have their own list "mdlnotfound".
* also added a few comments (looks silly, though)
add entry for autodownload in misc settings menu
sdl: use transparent icon

Looks like SDL_WM_SetIcon won't take half transparent pixels.