gdb: use bool in frame code

gdb: use bool in frame code

Change instances of int variables and return values used as boolean
values to use the bool type.

Shorten the comments of a few functions, because I think they go a bit
too much in implementation details, which appear out of date anyway.

Make other misc changes to the functions that are already being changed,
such as using nullptr instead of NULL, dropping `struct` keywords and
declaring variables when first used.

gdb/ChangeLog:

	* frame.h (frame_id_p): Return bool.
	(frame_id_artificial_p): Return bool.
	(frame_id_eq): Return bool.
	(has_stack_frames): Return bool.
	(get_selected_frame): Fix typo in comment.
	(get_frame_pc_if_available): Return bool.
	(get_frame_address_in_block_if_available): Return bool.
	(get_frame_func_if_available): Return bool.
	(read_frame_register_unsigned): Return bool.
	(get_frame_register_bytes): Return bool.
	(safe_frame_unwind_memory): Return bool.
	(deprecated_frame_register_read): Return bool.
	(frame_unwinder_is): Return bool.
	* frame.c (struct frame_info) <prev_arch::p>: Change type to
	bool.
	<this_id::p>: Likewise.
	<prev_p>: Likewise.
	(frame_stash_add): Return bool.
	(get_frame_id): Use bool.
	(frame_id_build_special) Use bool.
	(frame_id_build_unavailable_stack): Use bool.
	(frame_id_build): Use bool.
	(frame_id_p): Return bool, use true/false instead of 1/0.
	(frame_id_artificial_p): Likewise.
	(frame_id_eq): Likewise.
	(frame_id_inner): Likewise.
	(get_frame_func_if_available): Likewise.
	(read_frame_register_unsigned): Likewise.
	(deprecated_frame_register_read): Likewise.
	(get_frame_register_bytes): Likewise.
	(has_stack_frames): Likewise.
	(inside_main_func): Likewise.
	(inside_entry_func): Likewise.
	(get_frame_pc_if_available): Likewise.
	(get_frame_address_in_block_if_available): Likewise.
	(frame_unwinder_is): Likewise.
	(safe_frame_unwind_memory): Likewise.
	(frame_unwind_arch): Likewise.

Change-Id: I6121fa56739b688be79d73d087d76b268ba5a46a