Rename frame_stop_reason_string to unwind_stop_reason_to_string.

Rename frame_stop_reason_string to unwind_stop_reason_to_string.

  https://sourceware.org/ml/gdb-patches/2014-05/msg00721.html

This function is confusingly named, the "frame_" in the name implies it
somehow is frame dependent, when in reality the function just converts an
'enum unwind_stop_reason' value to a string.

gdb/ChangeLog:

	* frame.c (frame_stop_reason_string): Rename to ...
	(unwind_stop_reason_to_string): this.
	* frame.h (frame_stop_reason_string): Rename to ...
	(unwind_stop_reason_to_string): this.
	* stack.c (frame_info): Update call to frame_stop_reason_string.
	(backtrace_command_1): Likewise.
	* guile/scm-frame.c (gdbscm_unwind_stop_reason_string): Likewise.
	* python/py-frame.c (gdbpy_frame_stop_reason_string): Likewise.