Consolidate link delimiter functions

[splorp]

We currently use several separate functions to insert delimiters between posts, pages, archives, and images. These could be consolidated into a single, simplified function that uses a case switch and is able to pass through a custom delimiter glyph or entity.

The affected functions are:

delim_post_link()
delim_posts_link()
delim_comment_link()
delim_image_link()

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