Rework search_listings function

[apfohl]

In order to provide better encapsulation, the function search_listing() has to be rewritten. Right now, there are lots of printfs which are part of the user interface. That's not very elegant.

I would suggest that the function has only one responsibility. Take search options in the form of an adlist_t struct and return (by reference or value) a list of the search results. The UI part shall be completely moved to main.c.