dispatcher extensions

- distribute also Register and Subscribes to groups 20 and 30
  respectively
- for Invites introduce a backup group 2 to be used in case in group 1
  there are less than 3 available gateways
default.cfg - introduced DOS prevention

Use pike module to drop the SIP requests in case of a DOS
attack.
default.cfg:disable replys to Options from outside
registrar-role.cfg: DOS prevention

- reject Register to IP domain
- check for brute force attack - allow only two consecutive attempts to
  authenticate. If both failed block the account for 2 minutes.