(autosave . CDR)
This parameter determines what to do with modified buffers in closing
popup windows. CDR can be a t, 'ignore, a function or nil.
+ If t, no prompts. Just save them automatically (if they're
file-visiting buffers).
+ If 'ignore, no prompts, no saving. Just silently kill it.
+ If nil (the default), prompt the user what to do if the buffer is
file-visiting and modified.
+ If a function, the return value must return one of the other values.
It takes two arguments: the popup window and buffer.
