Add rpmtdSetTag() method for setting (or changing) container tag + type

- permit change on non-empty container to compatible type to allow things like
    headerGet(h, RPMTAG_FILENAMES, td, HEADERGET_EXT);
    rpmtdSetTag(td, RPMTAG_OLDFILENAMES);
    headerPut(h, td, HEADERPUT_DEFAULT);
- empty container can be set to any valid type

lib/rpmtd.c

@@ -191,6 +191,34 @@ char *rpmtdFormat(rpmtd td, rpmtdFormats fmt, const char *errmsg)
     return str;
 }
 
+int rpmtdSetTag(rpmtd td, rpmTag tag)
+{
+    assert(td != NULL);
+    rpmTagType newtype = rpmTagGetType(tag);
+    int rc = 0;
+
+    /* 
+     * Sanity checks: 
+     * - is the new tag valid at all
+     * - if changing tag of non-empty container, require matching type 
+     */
+    if (newtype == RPM_NULL_TYPE)
+	goto exit;
+
+    if (td->data || td->count > 0) {
+	if (rpmTagGetType(td->tag) != rpmTagGetType(tag)) {
+	    goto exit;
+	}
+    } 
+
+    td->tag = tag;
+    td->type = newtype & RPM_MASK_TYPE;
+    rc = 1;
+    
+exit:
+    return rc;
+}
+
 int rpmtdFromArgv(rpmtd td, rpmTag tag, ARGV_t argv)
 {
     int count = argvCount(argv);

lib/rpmtd.h

@@ -171,6 +171,16 @@ typedef enum rpmtdFormats_e {
  */
 char *rpmtdFormat(rpmtd td, rpmtdFormats fmt, const char *errmsg);
 
+/** \ingroup rpmtd
+ * Set container tag and type.
+ * For empty container, any valid tag can be set. If the container has
+ * data, changing is only permitted to tag of same type. 
+ * @param td		Tag data container
+ * @param tag		New tag
+ * @return		1 on success, 0 on error
+ */
+int rpmtdSetTag(rpmtd td, rpmTag tag);
+
 /** \ingroup rpmtd
  * Construct tag container from ARGV_t array.
  * Tag type is checked to be of string array type and array is checked