Fix mantis 2946 part C: when enabling sheields, only rebalance ETS if the shields were previously off

Also fix same issue in alter-ship-flag ( "no-shields" ... )

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10032 387891d4-d844-0410-90c0-e4c51a9137d3