Invert "was_single{,p}" logic to "is_list" -- we can actually prove a set of entries is actually a single entry.
Ensure GETMETADATA at least works in principle, against a single server.
annotate.c: fix for proxying SETANNOTATION
Save the external name of the mailbox as soon as we can, as part of mbentry.

The mbentry is being bounced around, but some places where we need the mbentry
name to be converted back to the 'external' style, we have no access to the
routines to do so.