(view-mode-enable): New function, split from view-mode-enter.
(view-mode-enter): Use view-mode-enable.
(view-mode-disable): New function, split from view-mode-exit.
(view-mode-exit): Use view-mode-disable.
(view-mode): Use view-mode-enable and view-mode-disable;
don't do all of what view-mode-enter and view-mode-exit do.
(change-major-mode-hook): Use view-mode-disable, not view-mode-exit.