Fix visiting and saving UTF-16 encoded XML files

Fix visiting and saving UTF-16 encoded XML files

* lisp/international/mule.el (sgml-xml-auto-coding-function):
When the 'enncoding' tag specifies a UTF-16 encoding, enforce
saving the buffer with BOM, per the XML spec.
(xml-find-file-coding-system): Recognize UTF-16 encodings with
BOM.  (Bug#35766)  (Bug#8282)
* lisp/international/mule-cmds.el (select-safe-coding-system):
Don't consider UTF-16 encodings with and without BOM as
"different", so as not to annoy users with redundant questions
about mismatch between the XML/SGML header and the selected
explicit encoding.