HeldDevice/HDFirearm defines how firmly it's being held instead of user

[MaximDude]

Problem:
The JointStrength property of HeldDevice and HDFirearm classes defines how firmly they are being held by whoever is currently holding them.
A physically weak actor (defined by visuals and/or properties) will handle the same amount of impulses from the device as a physically strong actor.
This does not make any sense and destroys unit type/class mechanics.

Solution:
Let actor handle how firmly he is holding devices by overriding JointStrength property of the device currently held.
Apply only to AHuman class (ACrab has a hard mounted HDFirearm so this is irrelevant).

This is groundwork for recoil rework in issue #7