Link Turret sharing xp

Since I have made the other turrets share xp with Engineers, I thought I would fix the problem with link turrets not sharing xp.
So now, if anyone with a link gun links to a link turret, the damage is boosted, the turret is healed and the person linking gets xp. Also supports link chains, with the xp split across all chain members.

Now the link turret is definitely too powerful for DC, and should not be in the configuration. However, tougher servers may want to use it, so the code can stay.