Add http-server-header as a last-ditch means to get httpd version

See http://seclists.org/nmap-dev/2013/q3/599 for justification.

scripts/http-server-header.nse

@@ -0,0 +1,51 @@
+local comm = require "comm"
+local string = require "string"
+local shortport = require "shortport"
+local nmap = require "nmap"
+
+description = [[
+Uses the HTTP Server header for missing version info. This is currently
+infeasible with version probes because of the need to match non-HTTP services
+correctly.
+]]
+
+---
+--@output
+-- PORT   STATE SERVICE VERSION
+-- 80/tcp open  http    Unidentified Server 1.0
+
+author = "Daniel Miller" 
+license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
+categories = {"version"}
+
+portrule = function(host, port)
+  -- Avoid running if -sV scan already got a match
+  if type(port.version) == "table" and (port.version.name_confidence > 3 or port.version.product ~= nil) then
+    return false
+  end
+  return shortport.http(host,port)
+end
+
+action = function(host, port)
+  local status, result = comm.tryssl(host, port,
+    "GET / HTTP/1.0\r\n\r\n",
+    {proto=port.protocol, timeout=5000})
+	
+	if (not status) then
+    return nil
+  end
+
+  local http_server = string.match(result, "\nServer:%s*(.-)\r?\n")
+  if http_server == nil then
+    return nil
+  end
+
+  port.version = port.version or {}
+  
+  if port.version.product == nil then
+    port.version.product = http_server
+  end
+  nmap.set_port_version(host, port, "hardmatched")
+	
+	return
+end