Add 'Tdisc' to libtags

Uses 'TPOS' for id3v2 tags, and 'DISCNUMBER' for vorbis comments (mostly
just relevant to flacs for our use case).

Fixes #181.

lib/libtags/id3v2.c

@@ -68,6 +68,8 @@ v2cb(Tagctx *ctx, char *k, char *v)
 			return 0;
 	}else if(strcmp(k-1, "COM") == 0 || strcmp(k-1, "COMM") == 0){
 		txtcb(ctx, Tcomment, k-1, v);
+	}else if(strcmp(k, "POS") == 0){
+		txtcb(ctx, Tdisc, k-1, v);
 	}else if(strcmp(k, "XXX") == 0){
 		k = v;
 		v += strlen(v) + 1;

lib/libtags/tags.h

@@ -16,6 +16,7 @@ enum
 	Ttitle,
 	Tdate, /* "2014", "2015/02/01", but the year goes first */
 	Ttrack, /* "1", "01", "1/4", but the track number goes first */
+	Tdisc,
 	Talbumgain,
 	Talbumpeak,
 	Ttrackgain,

lib/libtags/vorbis.c

@@ -14,6 +14,7 @@ static const struct {
 	{"artists", Tmultiartists},
 	{"albumartist", Talbumartist},
 	{"tracknumber", Ttrack},
+	{"discnumber", Tdisc},
 	{"date", Tdate},
 	{"replaygain_track_peak", Ttrackpeak},
 	{"replaygain_track_gain", Ttrackgain},