Set the default for VARIABLE to VALUE, and save it for future sessions.
Return VALUE.
If VARIABLE has a `custom-set' property, that is used for setting
VARIABLE, otherwise `set-default' is used.
If VARIABLE has a `variable-interactive' property, that is used as if
it were the arg to `interactive' (which see) to interactively read the value.
If VARIABLE has a `custom-type' property, it must be a widget and the
`:prompt-value' property of that widget will be used for reading the value.
If given a prefix (or a COMMENT argument), also prompt for a comment.
(fn VARIABLE VALUE &optional COMMENT)