(spam-summary-score-preferred-header): global preference

(spam-summary-score-preferred-header): global preference
for people who want to override the default SpamAssassin over
Bogofilter preference (when both are set)
(spam-necessary-extra-headers): add spam-use-bogofilter as an option
(spam-user-format-function-S): check spam-summary-score-preferred-header
(spam-extra-header-to-number): add X-Bogosity header parsing