Add selected list of categories option to function get_bookmarks()

[MichaelH]

In anticipation of the template function: wp_list_bookmarks (ticket:2587), add the option to return bookmarks belonging to a selected list of categories.

[MichaelH]

Selected categories for get_bookmarks

[MichaelH]

Patch to function get_bookmarks(), allows a parameter such as 'category=1,3,7' to return only bookmarks belonging to those three categories.

Also, changed so that $category = '' returns ALL categories (previously, $category = -1 returned ALL). This required adding line in get_links() to pass $category correctly when calling get_bookmarks().

Also fixed problem in get_links_list() to report bookmarks that only belong to a "child category" by adding &hierarchial=0 to the parameter list when calling get_categories().

[ryan]

(In [3658]) Retrieve bookmarks for multiple cats. Props MichaelH. fixes #2592