Don't write to XML output directly in nmap.cc. Spotted and fixed by
Michal Hrusecki (http://seclists.org/nmap-dev/2014/q3/180).
Add NSEdoc to re.lua

NSE libs must have @class and @name elements, or must have an NSEdoc
block preceding a call to stdnse.seeall, otherwise NSEdoc generation
breaks.
Add @usage to fcrdns.nse