Bug-fix: when enumerating a message header (returned from

MsgBase.get_msg_header or get_all_msg_headers), the message header object's
private data (used internally in these C source functions) would be freed and
NULL'd, apparenty as a form of optimization. This would cause some methods
which can accept a msg header object as an argument (e.g.
MsgBase.put_msg_header, bbs.show_msg/show_msg_header) to fail or behave
in strange ways.

Instead of freeing/NULLing the private data (and depending on that as an
indication that the header has been enuemrated), just set a member variable
indicating that the header has been enumerated (once) already.

This is the bug that has been tripping me up with my message lister JS mod
(see YouTube video). I can finally get that committed to CVS for testing now.
:-)