Structured script output.

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.

Target.cc

@@ -162,6 +162,10 @@ void Target::Recycle() {
 
 Target::~Target() {
   FreeInternal();
+  while (!scriptResults.empty()) {
+    scriptResults.front().clear();
+    scriptResults.pop_front();
+  }
 }
 
 void Target::FreeInternal() {