unit_shield_link.lua: 2 optimization

1) movement of non-networked shield-unit won't trigger re-linking of networked shield-units
2) movement of a networked shield-unit will only re-link current network, won't effect other network in the allyteam.

the 1st optimization is same as previous PR, but since previous PR was not included, its mentioned here for information.

Is tested :)

Did not change _G & SYNCED into RulesParam yet. Will do later (might take some time)