Add "root" disposition to attachments command.

An initial inline part was previously always skipped, for the good reasons outlined by @dgc in #217 (closed).

Unfortunately, some mail clients have started sending "attachments" as the root (and only) part, more and more lately. Although this may be bad practice, Mutt should support counting these as "attachments".

To do so without breaking existing configs, add a "root" disposition argument to the attachments command. This will count an initial inline part in a message or multipart container.

The existing code skipped counting for the "top-level" and recursive calls, so group both of these cases under "root".