Add option to use more than one confdir (RECOLL_EXTRACONFDIRS)
This change adds the possibility to run queries over more than one database by pointing the program at extra recoll configuration directories using the RECOLL_EXTRACONFDIR environment variable. This variable can contain space-separated recoll configuration directories (i.e. directories which contain `recoll.conf`) which are parsed to find out the indexed *topdirs* and the location of the database directory. The _topdirs_ are added to the directory tree, the databases are added to the `extradbs` list. When running a query over the entire tree (using `<all>`, the default) all databases are searched. When the query is limited to a subdirectory the searched set is limited to only those databases which cover the related _topdir_, thus reducing search time and overhead. The raison d'ètre for this change is to allow the web interface to be used to search a large index split over several databases, e.g. _fiction_, _nonfiction_ and _audio_. This in turn is used in the _recoll engine_ for the _Searx_ meta-search engine, see https://github.com/asciimoo/searx/pull/1257 . This is a further development of an earlier change I submitted to Github, most of which was merged but for the extra databases.