kill all flagged bufs in buffer menu without confirming each modified buffer

[Drew Adams]

WDOT of adding this feature to *Buffer List*:

Be able to hit some key (e.g. `D' or `C-x D') to delete all flagged buffers,
but instead of being queried separately to confirm deletion of each modified
buffer, be queried only once to confirm all of the killings.

This could be similar to what happens when you flag files for deletion in
Dired and then hit `x': the flagged buffers would be listed in a pop-up
buffer *Kill Modified Buffers*, and the message would be something like
"Kill modified buffers also?"

This would apparently require a change to the C source code for
`kill-buffer'.