(compilation-setup): Localize overlay-arrow-position.

(compilation-setup): Localize overlay-arrow-position.
(compilation-sentinel): Restructure code equivalently.
(compilation-next-error): Find message on same line after point if
not found before point.
(compile-mouse-goto-error): Restore function so that compilation
buffer need not be current and use compile-goto-error.
(compile-goto-error): Restore function.
(next-error): Set overlay-arrow-position.
(compilation-forget-errors): Don't localize already local
compilation-locs and remove FIXME about refontifying.