Fix bug using dwarf_next_unit to iterate over .debug_types

Fix bug using dwarf_next_unit to iterate over .debug_types

	* libdw_findcu.c (findcu_cb): Move earlier.
	(__libdw_intern_next_unit): Add new CU to search tree here...
	(__libdw_findcu): ... not here.

	* typeiter.c: New file.
	* run-typeiter.sh: New file.
	* testfile59.bz2: New file.
	* Makefile.am (noinst_PROGRAMS): Add typeiter.
	(TESTS): Add run-typeiter.sh.
	(EXTRA_DIST): Add run-typeiter.sh, testfile59.bz2.
	(typeiter_LDADD): New variable.

If you call dwarf_next_unit to iterate over .debug_types, then call
dwarf_offdie_types, you can see a failure if some earlier call
happened to call __libdw_intern_next_unit via dwarf_formref_die.

What happens is that __libdw_intern_next_unit updates the Dwarf's
next_tu_offset, but does not add the TU to the TU search tree.  So,
the call to dwarf_offdie_types does not find the TU in the tree, and
will not search any more, causing a failure.

This fix changes __libdw_intern_next_unit to add the TU to the search
tree, rather than relying on __libdw_findcu to do it.