lib: Reject multi-message mboxes and deprecate single-message mbox

lib: Reject multi-message mboxes and deprecate single-message mbox

Previously, we would treat multi-message mboxes as one giant email,
which, besides the obvious incorrect indexing, often led to
out-of-memory errors for archival mboxes.  Now we explicitly reject
multi-message mboxes.  For historical reasons, we retain support for
single-message mboxes, but official deprecate this behavior.