gdb/py: add some debugging to py-breakpoint.c

Adds some new debugging to python/py-breakpoint.c.

gdb/ChangeLog:

	* python/py-breakpoint.c (pybp_debug): New static global.
	(show_pybp_debug): New function.
	(pybp_debug_printf): Define.
	(PYBP_SCOPED_DEBUG_ENTER_EXIT): Define.
	(gdbpy_breakpoint_created): Add some debugging.
	(gdbpy_breakpoint_deleted): Likewise.
	(gdbpy_breakpoint_modified): Likewise.
	(_initialize_py_breakpoint): New function.

gdb/doc/ChangeLog:

	* python.texinfo (Python Commands): Document 'set debug
	py-breakpoint' and 'show debug py-breakpoint'.