WL#1735: Handler: "discover" database

There is a need for an interface for the handlers to use to list all existing
databases to perform various tasks on the existing and non-existing tables found
in them.

There already exist a function in sql_show.cc, make_db_list that returns a list
of local databases found in the local data directory. All that is needed is to
make this function public in sql_show.h.