Set/Get forceWalk (based on setCaptivity)

#1739 

Allows registering multiple sources of forceWalk.
Should increase compatibility with other scripts, but they'll need to use the new function.

Mission makers can do:

      [AGM_Player, "BrokenLeg", true] call AGM_Core_fnc_setForceWalkStatus;

And that person will forceWalk the entire mission.  Before they could drag and release a JerryCan to reset forceWalk.