Fix mantis 3020 (pilot stats issue)

Reset pilot stats when we're creating a new CSG file. Otherwise stats
from the current pilot or campaign will be inherited by the new pilot /
campaign

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@11011 387891d4-d844-0410-90c0-e4c51a9137d3