Correctly sort INBOX in base remote level
Only show remote mailboxes in dynamic view
For now, prohibit moving mailboxes and messages between accounts

This feature can be added later.  But it is not needed for the basic
usage right now.  (Moving messages *is* very useful, and will be
implemented, but will require a bit of extra work since we will need to
cache the data while moving).
[mms] Allow configurable access to remote accounts from within a session (Request #8077).

Going to call this completed - the basic functions seem to work well
and, at a minimum, it is stable enough to merge into imp_6_2, which
isn't that close to being released.