PR gdb/16483 - simplify "info frame-filters" output

PR gdb/16483 - simplify "info frame-filters" output

PR gdb/16483 notes that the output of "info frame-filters" is quite
voluminous.  In particular it prints an entry for each objfile, even if
only to say that the objfile does not have any associated frame filters.

I think it's better to only print output when there is a frame filter.
There's nothing worth doing with the no-frame-filter information, and
limiting the output makes it much more readable.

Built and regtested on x86-64 Fedora 23.

2016-06-23  Tom Tromey  <tom@tromey.com>

	PR gdb/16483:
	* python/lib/gdb/command/frame_filters.py
	(InfoFrameFilter.list_frame_filters): Rename to print_list.  Print
	nothing if no filters found.  Return value indicating whether
	filters were printed.
	(InfoFrameFilter.print_list): Remove.
	(InfoFrameFilter.invoke): Print message if no frame filters
	found.

2016-06-23  Tom Tromey  <tom@tromey.com>

	PR gdb/16483:
	* gdb.python/py-framefilter.exp: Add "info frame-filter" test
	before any filters are loaded.