register_dummy_frame_dtor: Permit multiple dtors

register_dummy_frame_dtor: Permit multiple dtors

Later patch needs two independent destructors for the same dummy_frame.
Therefore the registrar has been extended to an arbitrary number of
destructors.

gdb/ChangeLog
2015-05-13  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* dummy-frame.c (struct dummy_frame_dtor_list): New.
	(struct dummy_frame): Replace dtor and dtor_data by dtor_list.
	(remove_dummy_frame): Process dtor_list.
	(pop_dummy_frame): Process dtor_list.
	(register_dummy_frame_dtor): Maintain dtor_list.
	(find_dummy_frame_dtor): Handle dtor_list.
	* dummy-frame.h (register_dummy_frame_dtor, find_dummy_frame_dtor):
	Update comments.