New :type natnum for defcustom

* lisp/wid-edit.el (natnum): New widget type.  (Bug#15809)
* doc/lispref/customize.texi (Simple Types): Document it.

doc/lispref/customize.texi

@@ -594,6 +594,9 @@ want to take the time to work out a more specific type to use.
 @item integer
 The value must be an integer.
 
+@item natnum
+The value must be a nonnegative integer.
+
 @item number
 The value must be a number (floating point or integer).
 

etc/NEWS

@@ -3737,6 +3737,11 @@ instead of only becoming available after doing (for instance)
 ---
 ** ':safe' settings in 'defcustom' are now propagated to the loaddefs files.
 
++++
+** New ':type' for 'defcustom' for nonnegative integers.
+The new 'natnum' type can be used for options that should be
+nonnegative integers.
+
 +++
 ** ERT can now output more verbose test failure reports.
 If the 'EMACS_TEST_VERBOSE' environment variable is set, failure

lisp/wid-edit.el

@@ -3644,6 +3644,13 @@ match-alternatives: %S"
   :type-error "This field should contain an integer"
   :match-alternatives '(integerp))
 
+(define-widget 'natnum 'restricted-sexp
+  "A nonnegative integer."
+  :tag "Integer (positive)"
+  :value 0
+  :type-error "This field should contain a nonnegative integer"
+  :match-alternatives '(natnump))
+
 (define-widget 'number 'restricted-sexp
   "A number (floating point or integer)."
   :tag "Number"