Add headerPut() for adding data to headers through tag container

Add headerPut() for adding data to headers through tag container
- tag container isn't that big a win on add/append operations as it is on
  retrieval, just use the existing headerAddEntry() family to do the
  actual work
- headerPut() is a nice and short name, lifted from rpm5.org, API might
  differ
- flags to control operation, just whether append is permitted or not for now