ka, tka: refactor/fix ball carrier time scoring

ka, tka: refactor/fix ball carrier time scoring

ka: runs the ball carried think func every frame to facilitate other
features, which also allows the code to be simplified somewhat and makes
cvar g_keepaway_score_timeinterval redundant. See subsequent commits.

tka: as above, also implements fractional timepoints accumulation,
making g_tka_score_timepoints behaviour consistent with
g_keepaway_score_timepoints.