Set/Get forceWalk (based on setCaptivity)

#1739 (After dragging `forceWalk` will always be false no matter what the prior value was)

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.