(imenu-scanning-message): Support for bigger numbers.

(imenu-scanning-message): Support for bigger numbers.
(imenu--generic-function): Fixed probably a typo: named appeared
twice in an item. Put function after name and beg in a special
item because a normal item has name and beg (for orthogonality).
(imenu-add-to-menubar): First test to see if the mode supports imenu.
(imenu--menubar-select): Use the defined variable imenu-rescan-item.

(imenu-default-goto-function): New subroutine, broken out of `imenu'.
(imenu-default-goto-function): New variable.
(imenu): Made it use the above function, via that variable.