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).

src/dvdnav/dvdnav.h

@@ -588,7 +588,7 @@ dvdnav_status_t dvdnav_spu_language_select(dvdnav_t *self,
  * this is a descriptive string such as `THE_MATRIX' but sometimes is singularly
  * uninformative such as `PDVD-011421'. Some DVD authors even forget to set this,
  * so you may also read the default of the authoring software they used, like
- * `DVDVolume'.
+ * `DVDVolume' (see also dvdnav_get_volid_string).
  */
 dvdnav_status_t dvdnav_get_title_string(dvdnav_t *self, const char **title_str);
 
@@ -599,6 +599,19 @@ dvdnav_status_t dvdnav_get_title_string(dvdnav_t *self, const char **title_str);
  */
 dvdnav_status_t dvdnav_get_serial_string(dvdnav_t *self, const char **serial_str);
 
+/*
+ * Returns the VolumeIdentifier of the disc or NULL if it could
+ * not be obtained. The VolumeIdentifier might be latin-1 encoded
+ * (8bit unicode) null terminated and max 32 bytes (including '\0');
+ * or coded with '0-9','A-Z','_' null terminated and max 33 bytes
+ * (including '\0').
+ * See also dvdnav_get_title_string
+ *
+ * Note: The string is malloc'd so caller has to free() the returned
+ * string when done with it.
+ */
+const char * dvdnav_get_volid_string(dvdnav_t *self);
+
 /*
  * Get video aspect code.
  * The aspect code does only change on VTS boundaries.