Escape '%' in arguments to Thread:d.

A user reported this crash when scanning a target whose name contained
the '%' character:

NSE: Script Engine Scan Aborted.
An error was thrown by the engine: nse_main.lua:322: invalid capture index
stack traceback:
	[C]: in function 'gsub'
	nse_main.lua:322: in function 'd'
	nse_main.lua:377: in function 'start'
	nse_main.lua:912: in function 'run'
	nse_main.lua:1390: in function <nse_main.lua:1293>
	[C]: in ?

I'm not sure how a name with '%' got resolved, but I was able to
reproduce the crash by adding this line to /etc/hosts:
	127.0.0.1	a%40b
and then running
	./nmap --script=banner a%40b -d --top-ports 5

The gsub function recognizes "%d", where d is a digit, as a capture
index. The constructed string is then passed to print_debug, which is
like printf. Therefore we escape every occurrence of "%" twice, to get
"%%%%".