In bp_get_option(), check the buddypress()->site_options array first.

This is to prevent having to use get_blog_option() if we have already
fetched the option in question.

See #4913

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8188 cdf35c40-ae34-48e0-9cc9-0c9da1808c22