Generic get keywords function

I want to be able to get all keywords for a specific URL. 
 - While the function `yourls_get_duplicate_keywords()` exists, it returns NULL -- instead of a single keyword -- if `yourls_allow_duplicate_longurls()` is `false`. This pull refactors the keyword logic from `yourls_allow_duplicate_longurls()` into `yourls_get_keywords()` so a keyword list can be obtained regardless of the state of `YOURLS_UNIQUE_URLS`.
 - In a subsequent commit, I added the option to pass a sort (order by) parameter into the database call.

Honestly, I'm not sure why `yourls_get_duplicate_keywords()` runs this check.  It may make sense to update the caller of this function to do the desired checking then deprecate the function... but it's in place now for backwards compatibility.