The trackback_url functions are a bit haphazard. This is a patch to clean the up and bring them somewhat in line with the rest of the code.
trackback_url() accepts a $display parameter which is pointless when it's just using the get_trackback_url() function to do all the work.
get_trackback_url() set the trackback url to the default setting, then set it again to the correct URL if using permalinks. A minor optimization is to use an else{} to set it correctly in the first place and avoid the extra string work and function calls.
Minor optimizations, but still, we should try to keep things consistent and fast.