The customization type of recentf-max-saved-items is currently defined
as integer, which does not include nil in its domain.  However, setting
this variable to nil is supported in the code and also documented.

This patch changes the customization type to explicitly allow for the
variable to be set to nil through the Customization interface and
similar.  (Please note that I copied the type from save-place-limit in
order to be consistent.)