xmldoc.c: Fix dump of xml document

xmldoc.c:  Fix dump of xml document

The "xmldoc dump" cli command was simply concatenating xml documents
into the output file.  The resulting file had multiple "xml"
processing instructions and multiple root elements which is illegal.
Normally this isn't an issue because Asterisk has only 1 main xml
documentation file but codec_opus has its own file so if it's
downloaded and you do "xmldoc dump", the result is invalid.

* Added 2 new functions to xml.c:
    ast_xml_copy_node_list creates a copy of a list of children.
    ast_xml_add_child_list adds a list to an existing list.

* Modified handle_dump_docs to create a new output document and
  add to it the children from each input file.  It then dumps the
  new document to the output file.

Change-Id: I3f182d38c75776aee76413dadd2d489d54a85c07