Add sprinting

[4aiman]

I suggest adding sprinting ability to the minetest engine.
Here's the concept: Custom Hud + hunger mod + sprinting ability + some mobs.
Now for the "What's the purpose?" question:

The first one (a custom HUD) needed to graphically show that the player has food-o-meter, 'cause for now all that the hunger mod can do is send the player a message using Scripting API. That's not very nice, 'cause the player do not know how much time he has.

Hunger mod should be in the dependencies of the "stamina" mod or should be edited to include the sprinting ability: if the player is hungry he shouldn't be able to sprint. That's natural.

And here's the fun part:
In the minecraft sprinting is useless. There are NO mobs with velocity greater than that of the player. In Minetest all could be different. I suggest making it possible to create a rare powerful mob who can walk faster then the player. So, If it sees the player, it would chase him. And the sprinting would be one and only possibility to avoid him catch the player. But if the player is hungry he couldn't run and MUST engage with that mob. It's the matter of a few variables/constants to make that mob strong enough for any player want to avoid him.

To make all that possible I suggest adding a sprinting mode. To double press the forward key to sprint would be handy too. To stop sprinting and continue to walking when the player have bumped into the wall or entity would be natural. To stop sprinting in/under the water and on the sand/leaves would be natural too.

[xyzz]

https://github.com/celeron55/minetest_game/issues/1