Merge from /nmap-exp/patrick/nse-nsock-maintenance.

This is a maintenance fix for the NSE Nsock library binding. The patch focuses
on code correctness and simplicity. The patch also brings some initial updates
with an eye towards the upcoming Lua 5.2 release. See [1] for a post concerning
this branch.

[1] http://seclists.org/nmap-dev/2010/q3/710

nselib/ssh1.lua

@@ -25,13 +25,14 @@ require "openssl"
 --  @return packet_length, packet_length or nil
 --  the return is similar to the lua function string:find()
 check_packet_length = function( buffer )
+  if #buffer < 4 then return nil end
   local payload_length, packet_length, offset
   offset, payload_length = bin.unpack( ">I", buffer )
   local padding = 8 - payload_length % 8
   assert(payload_length)
-  packet_length = buffer:len()
-  if payload_length + 4 + padding > packet_length then return nil end
-  return packet_length, packet_length
+  local total = 4+payload_length+padding;
+  if total > #buffer then return nil end
+  return total, total;
 end
 
 --- Receives a complete SSH packet, even if fragmented
@@ -43,7 +44,7 @@ end
 --  @return status True or false
 --  @return packet The packet received
 receive_ssh_packet = function( socket )
-  local status, packet = socket:receive_buf(check_packet_length)
+  local status, packet = socket:receive_buf(check_packet_length, true)
   return status, packet
 end
 
@@ -76,7 +77,7 @@ fetch_host_key = function(host, port)
   padding = 8 - packet_length % 8
   offset = offset + padding
 
-  if padding + packet_length + 4 == data:len() then
+  if padding + packet_length + 4 == #data then
     -- seems to be a proper SSH1 packet
     local msg_code,host_key_bits,exp,mod,length,fp_input
     offset, msg_code = bin.unpack( ">c", data, offset )