Make nnimap support IMAP namespaces

Make nnimap support IMAP namespaces

* lisp/gnus/nnimap.el (nnimap-use-namespaces): Introduce new
server variable.
(nnimap-group-to-imap, nnimap-get-groups): Transform IMAP group
names to Gnus group name by stripping / prefixing personal
namespace prefix.
(nnimap-open-connection-1): Ask server for namespaces and store
them.

* lisp/gnus/nnimap.el (nnimap-request-group-scan)
(nnimap-request-create-group, nnimap-request-delete-group)
(nnimap-request-rename-group, nnimap-request-move-article)
(nnimap-process-expiry-targets)
(nnimap-request-update-group-status)
(nnimap-request-accept-article, nnimap-request-list)
(nnimap-retrieve-group-data-early, nnimap-change-group)
(nnimap-split-incoming-mail): Use nnimap-group-to-imap.
(nnimap-group-to-imap): New function to map Gnus group names to
IMAP folder names.  (Bug#21057)