If trying to create <mailbox><real separator>, give an error instead of working like <virtual separator>.
If trying to create <mailbox><real separator>, give an error instead of
working like <virtual separator>.

--HG--
branch : HEAD

dscho/dovecot,jwm/dovecot-notmuch,dscho/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,dscho/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,dscho/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,jkerihuel/dovecot,dscho/dovecot,jkerihuel/dovecot,jkerihuel/dovecot