Add an enhanced interface for loading, aka importing, headers

Add an enhanced interface for loading, aka importing, headers

- Unlike headerLoad(), headerImport() takes a blob size argument
  to allow sanity checking the size calculated from the blob itself
  against the "physical" passed-in blob size so its a bit safer.
  Note that header size is capped by various things - its not size_t.
- headerImport() also takes a flags argument to allow controlling
  various aspects of importing.
- Implement "take copy of blob" as a flag to headerImport(), push
  the copying into headerCreate() where we already know the blob
  size, avoiding the need to do double-calculations on headerCopyLoad()..
- headerLoad() and headerCopyLoad() are now just compat wrappers
  around the new interface.