Rename some frame unwind function parameters

Rename some frame unwind function parameters

I am currently working with these functions, and though this renaming
could help to reason about the code.  Some functions take a frame and
will return the value associated to that frame, others will return the
value associated to the previous frame.  Those usually conveniently
contain "unwind" in their name, but naming the variable next_frame
instead of frame helps remembering which frame we are dealing with.

I also included a little typo fix at the top of frame.h.

gdb/ChangeLog:

	* frame.c (frame_register_unwind): Change parameter name.
	(frame_unwind_register): Likewise.
	(frame_unwind_register_value): Likewise.
	(frame_unwind_register_signed): Likewise.
	(frame_unwind_register_unsigned): Likewise.
	* frame.h (frame_register_unwind): Likewise.
	(frame_unwind_register): Likewise.
	(frame_unwind_register_value): Likewise.
	(frame_unwind_register_signed): Likewise.
	(frame_unwind_register_unsigned): Likewise.
	(frame_unwind_arch): Likewise.