Ghosts can now possess borers if said borers have no ckey assigned.

Created by: Skullyton

This "fixes" #7541 (closed) by giving ghosts a chance to grab a borer during an infestation event if they get there before the valid squad, and in the off chance somebody gets pulled into a borer from egg that doesn't want it so they ghost, some other ghost who does want to be a borer can jump in instead.

Tested, shouldn't be any problems (Tried as a player observer, got in. Tried as a player who had just ghosted, couldn't get in)