Refactor: Extract function: xccdf_result_calculate_score
Refactor: Extract scoring functionalityu to separate module.

Generaly, we prefer not to move code around like this. However, there
are multiple reasons for this move:
 * Scoring code has characteristics of module
 * Scoring is independent of XCCDF Policy
 * XCCDF Policy module is big and hairy
 * We need to introduce functions like xccdf_result_recalculate_scores,
   it would be unfortunate to have these functions in XCCDF Policy
   module.