Merges the fields that are set in desc_to_merge into the fields in
desc. If replace_existing is FALSE, only fields in desc that
are not already set are affected. If TRUE, then fields that are
already set will be replaced as well.
If desc_to_merge is NULL, this function performs nothing.
Merges the fields that are set in desc_to_merge into the fields in desc. If replace_existing is FALSE, only fields in desc that are not already set are affected. If TRUE, then fields that are already set will be replaced as well. If desc_to_merge is NULL, this function performs nothing.