Ported the surface shield impact effects code from Wanderer branch, making it work on any type of shield and renaming the table entries to $Shield Impact Explosion and $Shield Impact Explosion Radius.

git-svn-id: svn://svn.icculus.org/fs2open/branches/zookeeper@10420 387891d4-d844-0410-90c0-e4c51a9137d3