libpacman: Implement FStringList::add.
libpacman: Don't use list pointers in chk_fileconflicts.
libpacman: Use FStringList::add where possible.
libpacman: Add FStringList::add(const FStringList &) specialisation.
doc: Document getters not returning NULL on empty lists anymore.
libpacman: Make Database::filter not return pointer.