WL#1587: encoding in xml dumps

As since 4.1 mysqldump is charset-aware it should put an encoding in xml dumps
(in --xml mode).

First line should look like

<?xml version="1.0" encoding="utf-8"?>

or

<?xml version="1.0" encoding="iso-8859-1"?>

for --default-character-set=latin1

the tricky part is where to take "standard" charset names from.
May be from SHOW CHARACTER SET ?
(then we should make sure that the first word in Description column is always
the correct IANA charset name as in www.iana.org/assignments/character-sets)