Separate non-post term counting from _update_post_term_count(). Introduce...

Separate non-post term counting from _update_post_term_count(). Introduce _update_generic_term_count(). The generic handler will be the default whenever the taxonomy is attached to an object type other than a post type (link, user). Otherwise the _update_post_term_count() handler will be the default. fixes #18986. see #17548.

git-svn-id: https://core.svn.wordpress.org/trunk@19035 1a063a9b-81f0-0310-95a4-ce76da25c4cd