[Enable mail splitting to work with non-ASCII nnml groups]

[Enable mail splitting to work with non-ASCII nnml groups]

* nnmail.el (nnmail-group-names-not-encoded-p): New variable.
(nnmail-split-incoming): Bind it.

* nnml.el (nnml-group-name-charset): New function.
(nnml-decoded-group-name): Use it; don't decode group name if
 nnmail-group-names-not-encoded-p is non-nil.
(nnml-encoded-group-name): New function.
(nnml-group-pathname): Inline nnml-decoded-group-name.
(nnml-request-expire-articles): Decode group name in message.
(nnml-request-delete-group): Ditto; bind file-name-coding-system to
 nnmail-pathname-coding-system.
(nnml-save-mail, nnml-active-number): Work with decoded group names and
 not decoded ones according to nnmail-group-names-not-encoded-p.
(nnml-generate-active-info): Use nnml-encoded-group-name.