PATCH: create and delete IMAP subfolders

[Peter Bloomfield <bloomfld eos ncsu edu>]

Balsa needs to be able to manage folder sets on IMAP servers, including
creating and deleting subfolders within an existing tree. This patch
provides those capabilities.

The patch messes with:
- libmutt, to get the right low-level hooks,
- libbalsa, to provide wrappers
- folder-conf and mailbox-conf, for the UI

It also changes balsa_mailbox_node_rescan, which didn't seem to handle
subtrees properly, and various IMAP functions in mailbox-node.c, to
populate more fields in IMAP mbnodes (parent is essential, and server and
subscribed are convenient).

Disclaimer: The patched Balsa works for me, with my Cyrus IMAP server, ...

balsa-imap-patch.gz