Add defense caps for mushroom terrain type, tweak some fluff

This Pull request adds negative defense modifiers to fungus, (thus establishing
a defense cap for mushroom mixed terrain types) for Dwarves,
Gryphon Riders, and Loyalist and Khalifate cavalry.

Pretty much it works along the lines I suggested here:
http://forums.wesnoth.org/viewtopic.php?f=15&t=40753&sid=5f6460f6567081ae62d5c9d0cf80357f#p573355

It also updates the "fluff" associated to fungus terrains to explain
in a different manner why dwarves and gryphon riders are weakened
here.

Feedback is very much welcome.

One extension of this which appeals to me is to also add negative modifiers
for the "swimmer" type, thus giving all mermen poor defense on mushrooms.
This might make swamp+mushroom a more interesting terrain type, as most
land units and mermen would be very weak and slow there but saurians 
would be fast and strong. However this change isn't strictly important for
balancing out the (Flat / Hill / Cave) + Mushroom so it isn't included here.