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/tftp-enum.nse

@@ -155,39 +155,8 @@ local check_file_present = function(host, port, filename)
   return FILE_NOT_FOUND
 end
 
---- Generates a random string of the requested length. This can be used to check how hosts react to
--- weird username/password combinations.
--- @param length (optional) The length of the string to return. Default: 8.
--- @param set (optional) The set of letters to choose from. Default: upper, lower, numbers, and underscore.
--- @return The random string.
-local function get_random_string(length, set)
-  if (length == nil) then
-    length = 8
-  end
-
-  if (set == nil) then
-    set = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_"
-  end
-
-  local str = ""
-
-  -- Seed the random number, if we haven't already
-  if (not (nmap.registry.oracle_enum_users) or not (nmap.registry.oracle_enum_users.seeded)) then
-    math.randomseed(os.time())
-    nmap.registry.oracle_enum_users = {}
-    nmap.registry.oracle_enum_users.seeded = true
-  end
-
-  for i = 1, length, 1 do
-    local random = math.random(#set)
-    str = str .. string.sub(set, random, random)
-  end
-
-  return str
-end
-
 local check_open_tftp = function(host, port)
-  local random_name = get_random_string()
+  local random_name = stdnse.generate_random_string(8, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_")
   local ret_value = check_file_present(host, port, random_name)
   if (ret_value == FILE_FOUND or ret_value == FILE_NOT_FOUND) then
     return true