gdb: Avoid unneeded calls to parse_frame_specification.

gdb: Avoid unneeded calls to parse_frame_specification.

Within the stack command there are a couple of places where fixed
strings are passed into functions that are really intended for
processing user input.  These fixed strings are then processed and the
result returned.

Given that the input strings in these cases are fixed, and are always
"0", then the result will always be the same, the current frame.  By
switching to using get_current_frame instead the code can be simplified,
and the intention of the code is clearer.

gdb/ChangeLog:

	* stack.c (parse_frame_specification): Delete.
	(parse_frame_specification_1): Rename to
	parse_frame_specification.
	(frame_info): Use parse_frame_specification.
	(select_frame_command): Likewise.
	(return_command): Use select_frame and print_stack_frame rather
	than frame_command and select_frame_command.
	(func_command): Use get_current_frame rather than
	parse_frame_specification.