Increased movement speed from gear shouldn't stack

[isotor]

I have:

• [x] searched existing issues (http://github.com/darkstarproject/darkstar/issues/) to see if the issue I am posting has already been addressed or opened by another contributor
• [x] checked the commit log to see if my issue has been resolved since my server was last updated

Client Version (type /ver in game) : 30190328_2

Source Branch (master/stable) : master

Additional Information (Steps to reproduce/Expected behavior) :

As much as I like rocking around town in my Trotters/Crimson Cuisses/Aketon for a base +36% IMS shouldn't be possible. Excerpt from the relevant wiki page:

• Movement speed modifying equipment will stack with all forms of movement speed effects, other than Chocobo and Cheer.
• Boosts to movement from multiple pieces of equipment will not stack with each other. The largest boost is the one the player will receive.
• However, movement speed decrease from equipment will stack.

[TeoTwawki]

Preference for resolution here: 2 modifiers, one that stacks and one that doesn't. Right now we are using the same mod for both, with negative values for speed decrease. MOD_MOVE <- exists MOD_MOVE_NOSTACK <- doesn't