pdb: _bfd_generic_close_and_cleanup

Every format that might appear inside a generic archive needs to call
_bfd_generic_close_and_cleanup, so that the archive element lookup
htab can be tidied on closing an element.  Otherwise you get stale
entries in the htab pointing at freed and perhaps reused memory,
resulting in segfaults when the archive is closed.

Calling _bfd_generic_close_and_cleanup on close means tdata needs to
be set up too, since pdb claims to be of format bfd_archive.

	* pdb.c (pdb_close_and_cleanup): Define as
	_bfd_generic_close_and_cleanup.
	(pdb_archive_p): Set up tdata for bfd_archive format.