Show the tag number for unknown tags in --xml output

--xml is only really useful for diagnostics, and for that purpose in
particular, showing "(unknown)" text instead of the tag number
for unknown tags is just offensively dumb. No more!

lib/headerfmt.c

@@ -772,6 +772,12 @@ static char * singleSprintf(headerSprintfArgs hsa, sprintfToken token,
 
 	    if (isxml) {
 		const char * tagN = rpmTagGetName(spft->u.tag.tag);
+		char *tagval = NULL;
+
+		if (rstreq(tagN, "(unknown)")) {
+		    rasprintf(&tagval, "[%u]", spft->u.tag.tag);
+		    tagN = tagval;
+		}
 
 		need = sizeof("  <rpmTag name=\"\">\n") - 1;
 		if (tagN != NULL)
@@ -782,6 +788,8 @@ static char * singleSprintf(headerSprintfArgs hsa, sprintfToken token,
 		    te = stpcpy(te, tagN);
 		te = stpcpy(te, "\">\n");
 		hsa->vallen += (te - t);
+
+		free(tagval);
 	    }
 
 	    t = hsaReserve(hsa, need);