Some string optimizations in NSE

Changes fall into these categories:

1. Avoid pathological string building. Loops over x = x .. "foo" can
become very slow. Instead, use strbuf.lua, table.concat, or just one
continuous concatenation; a = x .. y .. z is one operation, better than
a = x .. y; a = a .. z

2. Use hex-escaped strings instead of string.char. I find this more
readable in many cases, and it avoids a table lookup and function call.

3. Don't duplicate code. A few libraries and scripts had re-implemented
stdnse.generate_random_string or openssl.rand_bytes.

nselib/mssql.lua

@@ -1732,7 +1732,7 @@ LoginPacket =
   library = "mssql.lua",
   locale = "",
   database = "master", --nil,
-  MAC = string.char(0x00,0x00,0x00,0x00,0x00,0x00), -- should contain client MAC, jTDS uses all zeroes
+  MAC = "\x00\x00\x00\x00\x00\x00", -- should contain client MAC, jTDS uses all zeroes
 
   new = function(self,o)
     o = o or {}