returns a linked database if the short & full names allow to find any

1. Look at descriptions for getFileName() & getFullFileName() for comments about how the file search works
2. It can be either TG database, TD database or a database of other types
3. All databases ordered by references are loaded and stored into the internal cash of the host database since they are ordered for the first time
4. All returned databases are opened for read only
5. Returns zero if no database is found