[Pine-info] Clear input buffer when putting up postponed message prompt

[Matt Ackeret]

Can pine clear the buffer when it brings up a composition postponed
message?

I just did it a minute ago, and I did it when starting to send this
message!

For this message, I typed cpine, so the 'n' answered the continue postponed
message prompt.. (before, the 'y' in someone's name answered yes.. so of
course I had to postpone again)

Maybe if it notices a few missed keys, it can even delay for a second so
the user doesn't keep typing.. but usually it's me typing stuff before
the postponed prompt even comes up.