Detached weapon behind back

BUR #50338 is pending approval.

mass update favgroup:43119 -> detached_weapon_behind_back

I still think we need a tag for this situation.
When a weapon is behind the back, but not holding behind back, not worn on the back, also not weapon background.
Just like floating weapon or planted weapon behind back are common types.

I don't think we should be lumping planted weapons behind back and floating weapons behind back together, though I'd agree on a new tag especially for the latter.