Minor bug fixes to --os-shell (altought web backdoor functionality still to be reviewed).

Minor common library code refactoring.
Code cleanup.
Set back the default User-Agent to sqlmap for comparison algorithm reasons.
Updated THANKS.

lib/techniques/inband/union/test.py

@@ -175,8 +175,9 @@ def __unionTestByNULLBruteforce(comment):
 
 
 def __unionTestByOrderBy(comment):
-    columns = None
-    value   = None
+    columns     = None
+    value       = None
+    prevPayload = ""
 
     for count in range(1, 51):
         query        = agent.prefixQuery(" ORDER BY %d" % count)