Add an XML_PARSE_NOXXE flag to block all entities loading even local

For https://bugzilla.gnome.org/show_bug.cgi?id=772726

* include/libxml/parser.h: Add a new parser flag XML_PARSE_NOXXE
* elfgcchack.h, xmlIO.h, xmlIO.c: associated loading routine
* include/libxml/xmlerror.h: new error raised
* xmllint.c: adds --noxxe flag to activate the option