Fix escaping null characters in mailbox names.

Fix escaping null characters in mailbox names.

The null character can not be used in form elements. However, we use the
null character in mailbox names to indicate mailboxes/elements that do
not live on the IMAP server - necessary because null is the only
character technically not allowed for IMAP mailboxes.  So we need to
manually call IMP::formMbox() when creating forms to escape mailbox
names (and then when reading forms to unescape the names - but most of
the time this will be done automatically by
IMP::setCurrentMailboxInfo()).