* mm-util.el (mm-charset-synonym-alist): Improve doc string.

* mm-util.el (mm-charset-synonym-alist): Improve doc string.
(mm-charset-override-alist): New variable.
(mm-charset-to-coding-system): Use it.
(mm-codepage-setup): New helper function.
(mm-charset-eval-alist): New variable.
(mm-charset-to-coding-system): Use mm-charset-eval-alist.  Warn
about unknown charsets.  Add allow-override.  Use
`mm-charset-override-alist' only when decoding.
(mm-detect-mime-charset-region): Use :mime-charset.

* mm-bodies.el (mm-decode-body, mm-decode-string): Call
`mm-charset-to-coding-system' with allow-override argument.