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.

scripts/nping-brute.nse

@@ -34,14 +34,6 @@ categories = {"brute", "intrusive"}
 
 portrule = shortport.port_or_service(9929, "nping-echo")
 
-local function randombytes(x)
-  local bytes = ""
-  for i = 1, x do
-    bytes = bytes .. bin.pack("C", math.random(0x00, 0xff))
-  end
-  return bytes
-end
-
 local function readmessage(socket, length)
   local msg = ""
   while #msg < length do
@@ -103,8 +95,8 @@ Driver =
     local NEP_CLIENT_MAC_ID = "NEPkeyforMACClient2Server"
 
     local now = nmap.clock()
-    local seqb = randombytes(4)
-    local cnonce = randombytes(32)
+    local seqb = openssl.rand_bytes(4)
+    local cnonce = openssl.rand_bytes(32)
     local nonce = snonce .. cnonce
     local enckey = self:nepkey(password, nonce, NEP_CLIENT_CIPHER_ID)
     local mackey = self:nepkey(password, nonce, NEP_CLIENT_MAC_ID)