Move unwind reasons to an external .def file

Move unwind reasons to an external .def file

gdb/
	* frame.c (frame_stop_reason_string): Rewrite using
	unwind_stop_reasons.def.
	* frame.h (enum unwind_stop_reason): Likewise.
	* python/py-frame.c (gdbpy_initialize_frames): Likewise.
	(gdbpy_frame_stop_reason_string): Use new enum unwind_stop_reason
	constants for bound-checking.
	* unwind_stop_reasons.def: New file.
	* stack.c (backtrace_command_1): Handle UNWIND_FIRST_ERROR as an alias
	instead of a distinct value.

doc/
	* gdb.texinfo ((Frames In Python): Document
	gdb.FRAME_UNWIND_FIRST_ERROR contant.