Add a $taxonomy argument to each of the adjacent post functions.

Each took an array of category (IDs) when to search. Those can now be term IDs and each function now has $taxonomy = 'category' as an optional argument.

Functions affected: get_previous_post(), get_next_post(), get_adjacent_post(), get_adjacent_post_rel_link(), adjacent_posts_rel_link(), next_post_rel_link(), prev_post_rel_link(), get_boundary_post(), get_previous_post_link(), previous_post_link(), get_next_post_link(), next_post_link(), get_adjacent_post_link(), adjacent_post_link().

props ethitter.
finally fixes #17807.


git-svn-id: https://develop.svn.wordpress.org/trunk@25959 602fd350-edb4-49c9-b593-d223f7449a82