Consolidate pagination link functions

[splorp]

We currently use several separate functions to test whether pagination links should be shown on posts, pages, archives, and images. These could be consolidated into a single, simplified function that uses a case switch.

The affected functions are:

show_post_link_nav()
show_posts_link_nav()
show_comments_link_nav()
show_image_link_nav()

The new function should be named using the tersus_ prefix and use a function_exists wrapper similar to other pluggable theme functions.