autofs: refactor parse_options()

Seperate out parts of parse_options() that will match better the
individual option processing used in the mount API to further simplify
the upcoming conversion.

Signed-off-by: Ian Kent <raven@themaw.net>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>
Message-Id: <20230922041215.13675-6-raven@themaw.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>