MageKing17's patch for callsigns not resetting when the mission is cleared (Mantis #2934)

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10863 387891d4-d844-0410-90c0-e4c51a9137d3
from MageKing17: choose alt_names and callsigns by array index instead of substring; fixes Mantis #2934

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10864 387891d4-d844-0410-90c0-e4c51a9137d3