Scripts may now return a key–value table, or such a table in addition to
a string. The table will be automatically formatted for normal output
and will appear as a hierarchy of elements in XML output.
Some history and discussion of this development can be found at
https://secwiki.org/w/Nmap/Structured_Script_Output.
This is a merge of r29484:29569 from /nmap-exp/david/xml-output.