Make "bt N" print correct number of frames when using a frame filter

PR python/16497 notes that using "bt" with a positive argument prints
the wrong number of frames when a frame filter is in use.  Also, in this
case, the non-frame-filter path will print a message about "More stack
frames" when there are more; but this is not done in the frame-filter
case.

The first problem is that backtrace_command_1 passes the wrong value
to apply_ext_lang_frame_filter -- that function takes the final
frame's number as an argument, but backtrace_command_1 passes the
count, which is off by one.

The solution to the second problem is to have the C stack-printing
code stop at the correct number of frames and then print the message.

Tested using the buildbot.

ChangeLog
2018-02-26  Tom Tromey  <tom@tromey.com>

	PR python/16497:
	* stack.c (backtrace_command_1): Set PRINT_MORE_FRAMES flag.  Fix
	off-by-one in py_end computation.
	* python/py-framefilter.c (gdbpy_apply_frame_filter): Handle
	PRINT_MORE_FRAMES.
	* extension.h (enum frame_filter_flags) <PRINT_MORE_FRAMES>: New
	constant.

2018-02-26  Tom Tromey  <tom@tromey.com>

	PR python/16497:
	* gdb.python/py-framefilter.exp: Update test.