Introduce htab_up and use gdbpy_enter in py-framefilter.c

Introduce htab_up and use gdbpy_enter in py-framefilter.c

This introduces a new "htab_up" typedef, which is a std::unique_ptr
that can call htab_delete.  Then it changes some code in
py-framefilter.c to use both gdbpy_enter and the new htab_up.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* utils.h (htab_deleter): New struct.
	(htab_up): New typedef.
	* python/py-framefilter.c (gdbpy_apply_frame_filter): Use
	gdbpy_enter, gdbpy_ref, htab_up.