Complete doxygen_utility.vim.

Add:
	vim/(bundle/doxygen_utility/plugin > scripts)/doxygen_utility.vim
		-Move the plugin from `bundle/` to `scripts/`, because the latter's
		associated with others' scripts and plugins.

	vim/scripts/doxygen_utility.vim
		-Add `Doxygen` directive abbreviations.

	vim/ftplugin/c.vim
		-Move `Doxygen` syntax matches from the c ftplugin file to
		`doxygen_utility.vim`.