Sync with `t-gnus-6_14'

Sync with `t-gnus-6_14'
-----------------------
* dgnushack.el (dgnushack-texi-format): Use `output-coding-system' instead of
`coding-system-for-write' when old Mule is used.

Sync with Gnus
--------------
* nndraft.el (nndraft-request-replace-article): Replace `mm-text-coding-system'
with `mail-source-text-coding-system'; Replace `mm-auto-save-coding-system'
with `message-draft-coding-system'.

* mail-source.el (mail-source-fetch-maildir): Replace `mm-text-coding-system'
with `mail-source-text-coding-system'.
(mail-source-text-coding-system): New variable.

* others...