Turn gdbpy_ref into a template

This turns gdbpy_ref into a template class, so that it can be used to
wrap subclasses of PyObject.  The default argument remains PyObject;
and this necessitated renaming uses of "gdbpy_ref" to "gdbpy_ref<>".

gdb/ChangeLog
2017-02-10  Tom Tromey  <tom@tromey.com>

	* python/py-ref.h (gdbpy_ref_policy): Now a template.
	(gdbpy_ref): Now a template; allow subclasses of PyObject to be
	used.
	* python/py-arch.c, python/py-bpevent.c, python/py-breakpoint.c,
	python/py-cmd.c, python/py-continueevent.c, python/py-event.c,
	python/py-exitedevent.c, python/py-finishbreakpoint.c,
	python/py-framefilter.c, python/py-function.c,
	python/py-inferior.c, python/py-infevents.c,
	python/py-linetable.c, python/py-newobjfileevent.c,
	python/py-param.c, python/py-prettyprint.c, python/py-ref.h,
	python/py-signalevent.c, python/py-stopevent.c,
	python/py-symbol.c, python/py-threadevent.c, python/py-type.c,
	python/py-unwind.c, python/py-utils.c, python/py-value.c,
	python/py-varobj.c, python/py-xmethods.c, python/python.c,
	varobj.c: Change gdbpy_ref to gdbpy_ref<>.