fix Mantis #2796 (capships always have an EMP time of 7.5 seconds) by adding a weapon flag to allow the full value of emp_time to be used

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@9540 387891d4-d844-0410-90c0-e4c51a9137d3