Whether to add a newline at end of file, in certain major modes.
Those modes set `require-final-newline' to this value when you enable them.
They do so because they are often used for files that are supposed
to end in newlines, and the question is how to arrange that.
A value of t means do this only when the file is about to be saved.
A value of `visit' means do this right after the file is visited.
A value of `visit-save' means do it at both of those times.
Any other non-nil value means ask user whether to add a newline, when saving.
A value of nil means do not add newlines. That is a risky choice in this
variable since this value is used for modes for files that ought to have
final newlines. So if you set this to nil, you must explicitly check and
add a final newline, whenever you save a file that really needs one.