Add 3rd person view for the local player

[Carsten]

This patch replaces the traditional first-person view of the local player with a third-person camera.

The third-person camera can be modified via the new console-variable cl_3rdPerson that determines the 3rd-person camera mode that should be used:

1. The first is aiming at the player directly. It's the most simple one, but cannot easily be used in shooters, since you cannot directly see what you're aiming at.
2. The second one is aiming over the players head, similar to Tomb Raider.
3. The third is an over-the-shoulder view, as you see it in many current shooters.

Original code and patch by .red, see <http://www.cafu.de/forum/viewtopic.php?f=8&t=760> for details.