Adding "interceptable", "turret interceptable", and "fighter interceptable" weapon flags.

These allow munitions to be attacked by the AI without needing to set the "bomb" flag; "turret interceptable" means it can be attacked by turrets, "fighter interceptable" means it can be shot down by fighters, and "interceptable" sets both flags simultaneously. Requested by Rheyah.

git-svn-id: svn://svn.icculus.org/fs2open/trunk@11171 387891d4-d844-0410-90c0-e4c51a9137d3