unit_shield_link.lua: fix shield did not re-link after being EMP-ed or disarmed.
This is bug from previous optimization (OPS). It accidentally skipped updates for shieldUnits with NO_LINK (NO_LINK was assigned for EMPed shieldUnits while a table with 1 entry is for lonely shieldUnits)
unit_shield_link.lua: some tiny cleanup to make it a bit readable.
No logic changes is intended, just renamed functions and encapsulate some stuff.
Merge pull request #69 from xponen/shieldLink_bug

fix Shield link do not re-link after EMP-ed/Disarmed. Need to deploy VERSION{v1.2.7.5}