Add a secure command to renegotiate TLV encryption

This gives us the ability to force TLV encryption if for some reason
it's not already in place, and it means we can renegotiate a new key on
the fly if we want to.

lib/rex/post/meterpreter/client_core.rb

@@ -701,6 +701,10 @@ def migrate(target_pid, writable_dir = nil, opts = {})
     return true
   end
 
+  def secure
+    client.tlv_enc_key = negotiate_tlv_encryption
+  end
+
   #
   # Shuts the session down
   #

lib/rex/post/meterpreter/ui/console/command_dispatcher/core.rb

@@ -60,6 +60,7 @@ def commands
       'use'                      => 'Deprecated alias for "load"',
       'load'                     => 'Load one or more meterpreter extensions',
       'machine_id'               => 'Get the MSF ID of the machine attached to the session',
+      'secure'                   => '(Re)Negotiate TLV packet encryption on the session',
       'guid'                     => 'Get the session GUID',
       'quit'                     => 'Terminate the meterpreter session',
       'resource'                 => 'Run the commands stored in a file',
@@ -319,6 +320,12 @@ def cmd_sessions(*args)
     end
   end
 
+  def cmd_secure
+    print_status('Negotiating new encryption key ...')
+    client.core.secure
+    print_good('Done.')
+  end
+
   def cmd_background_help
     print_line('Usage: background')
     print_line