cxxll::rpmfi_handle: Add class
cxxll::rpm_parser_state: Implement using cxxll::rpmfi_handle
cxxll::rpm_file_info::linkto: Add member
cxxll::rpm_parser_state: Set linkto member
database::add_symlink: Obtain link target from rpm_file_info

This is always available even for ghost symlinks.
SQL schema: Add flags column to symboldb.file and symboldb.directory