Provide a way of obtaining the dvd volume id

In Kodi we fill a dvd label using dvdnav_get_serial_string to provide a string for skins to use:

However in our version of libdvdnav XBMC hack the filling of dvd->name by also obtaining the info from the DVD volume if the title string could not be obtained: https://github.com/xbmc/libdvdnav/commit/48607e689662f21210905ac75ac21424f764fcde

I don't think this is a clean solution as it violates the single responsibility principle and mixes up different data on the same source of information. Hence this PR adds dvdnav_get_volid_string as an alternative giving that the client/application can fallback to it if dvdnav_get_serial_string fails. We could already do this using dvdreader only but that would require going via DVDOpen for a disc that is already open (the dvd_reader_t pointer is private to the VM).