Update some xml.cc comments and add encoding and standalone declarations

Explanatory comments within, but this came from reading the XML 1.0
spec. See some history:

http://seclists.org/nmap-dev/2013/q4/48 - Added DOCTYPE
http://seclists.org/nmap-dev/2014/q2/82 - broke Zenmap
http://seclists.org/nmap-dev/2014/q3/331 - reduced DOCTYPE