unit_shield_link.lua: 2 optimization

1) movement of non-connected shield-unit won't trigger re-linking of other shield-units (previous PR)
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,is mentioned again for fun.

Is tested :)

Finished change _G & SYNCED into RulesParam