muse-docbook: Output an XML tag with encoding at the beginning of document.

muse-docbook: Output an XML tag with encoding at the beginning of document.

* lisp/muse-docbook.el (muse-docbook-header): Add <xml> tag at beginning
  of document.
  (muse-docbook-encoding-default): New option that determines which Emacs
  buffer encoding to use by default in Muse DocBook files if none is
  found.
  (muse-docbook-charset-default): New option that determines the DocBook
  XML charset to use if no translation is found in
  muse-docbook-encoding-map.
  (muse-docbook-encoding-map): New option alist that maps an emacs coding
  system to its associated DocBook coding system.
  (muse-docbook-transform-content-type): New function that determines the
  DocBook XML encoding to use based on the contents of
  muse-docbook-encoding-map.
  (muse-docbook-encoding): New function that calls
  muse-docbook-transform-content-type.
  (muse-docbook-finalize-buffer): New function that sets the buffer file
  coding system to the value of muse-docbook-encoding-default, but only
  if the buffers contents have no special characters.
  ("docbook"): Include :after tag that calls muse-docbook-finalize-buffer.
git-archimport-id: mwolson@gnu.org--2005/muse--main--1.0--patch-122