Handle HEADERFLAG_SORTED bit correctly in headerUnsort()

Handle HEADERFLAG_SORTED bit correctly in headerUnsort()
- Within rpm there's exactly one caller of headerUnsort() which has
  kinda taken care of re-sorting the header on exit, but only if it
  returns successfully, meaning the header sort status could've been
  left in inconsistent state with implications on consequent
  operations on that header. Also this is part of the public API
  yet callers have no chance of adjusting the flag when they call it
  (and why should they).
- Also dont bother sorting if the header is already in unsorted state.