PR python/19293 - invalidate frame cache when unwinders change

PR python/19293 - invalidate frame cache when unwinders change

PR python/19293 notes that when a Python unwinder is disabled, the
frame cache is not invalidated.  This means that disabling an unwinder
doesn't have any immediate effect -- but in my experience it's often
the case that I want to enable or disable an unwinder in order to see
what happens.

This patch adds a new gdb.invalidate_cached_frames function and
arranges for the relevant bits of library code to call it.  I've only
partially documented this function, considering a warning sufficient
without going into all the reasons ordinary code should not call it.
The name of the new function was taken from a comment in frame.h next
to reinit_frame_cache.

No new test as I think the updates to the existing test are sufficient
to show that the code is working as intended.

Built and regtested on x86-64 Fedora 23.

2016-07-12  Tom Tromey  <tom@tromey.com>

	PR python/19293:
	* python/lib/gdb/command/unwinders.py (do_enable_unwinder): Call
	gdb.invalidate_cached_frames.
	* python/lib/gdb/unwinder.py (register_unwinder): Call
	gdb.invalidate_cached_frames.
	* python/python.c (gdbpy_invalidate_cached_frames): New function.
	(python_GdbMethods): Add entry for invalidate_cached_frames.

2016-07-12  Tom Tromey  <tom@tromey.com>

	PR python/19293:
	* python.texi (Frames In Python): Document
	gdb.invalidate_cached_frames.

2016-07-12  Tom Tromey  <tom@tromey.com>

	PR python/19293:
	* gdb.python/py-unwind-maint.exp: Update tests.