get confirmation before killing a modified buffer

[lewang]

use kill-buffer-query-functions instead of kill-buffer-hook.

[stsquad]

I'm not if this has broken with the re-factoring but a couple of points:

C-c C-c seems to prompt, surely if I say done I mean yes save and return
C-x C-c works as expected when the buffer is modified

However in both cases the frame is left up, it should be destroyed.

I was half-way through trying to get the iterative edit feature to work so I'll see if I can get that fixed up now.