use user_breakpoint_p in python code

I noticed that bppy_get_visibility and gdbpy_breakpoint_created
implemented their own visibility checks, but subtly different from
user_breakpoint_p.  I think the latter is more correct, and so changed
the Python code to use it.

I suspect there isn't a decent way to test this, so no new test.

Built and regtested on x86-64 Fedora 23.

2016-07-13  Tom Tromey  <tom@tromey.com>

	* python/py-breakpoint.c (bppy_get_visibility)
	(gdbpy_breakpoint_created): Use user_breakpoint_p.