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
Add custom description of demos

Custom description is displayed in "Watching demo" and "Play demo" menu.
Fix spectating after disconnection of spectated player

Before if you watched player from the first spectating position
(SM_FOLLOW1ST), and that player disconnected, then you switched
automatically to another player, but also to second spectating position
(SM_FOLLOW3RD), instead of stay in SM_FOLLOW1ST - this commit fixes that.
Allow persistent spectating concrete player in multiplayer

For spectatepersistent = 1 that is always enabled (that means, that
spectated player isn't changed after his death), for 0  - only in
non-arena modes, except of demos.
"if(!available.length()) return f;" is introduced, in order to, in arena
modes, doesn't happen switching to SM_FLY, when last alive players dies
in the same moment and there is no winner of the round.
Fix output messages in console

Fixes:
1. show time remaining in system console after each minute;
2. correct form of minute(s);
3. show info "everyone died!" instead of "team SPECTATOR has won
the round!" in demo mode;
4. show correct info to which team (name) was player forced, when player1
is spectator.
Add the clock to scoreboard and change clockdisplay

Commit shows time in format mm:ss in scoreboard, instead of
"X minutes remaining" info.
It also enables the clock default on the HUD and adds "clockcount"
variable which let to choose between counting forward or backward
in the clocks.

DOCREF clockdisplay clockcount
Cleanup maprot