(autoload-generated-file): Interpret names relative to current dir

(autoload-generated-file): Interpret names relative to current dir
for file-local settings.
(autoload-generate-file-autoloads): Add `outfile' arg.
(update-directory-autoloads): Use it to directly call
autoload-generate-file-autoloads instead of going through
update-file-autoloads so we avoid redundant searches and so we can know
the set of buffers changed so we can save them all.