Ignore unrecognised mailbox roles

[robn]

RFC8621 says mailbox roles MUST come from this list: https://www.iana.org/assignments/imap-mailbox-name-attributes/imap-mailbox-name-attributes.xhtml

As a safety mujmap should either ignore any non-null roles that aren't on the list or, maybe, only permit ones that we know we can deal with.

(servers might accidentally present non-standard roles and we don't want to assume that they're just normal mailboxes lest we make modifications that cause additional actions on the server).