baresip: add procd init script

  - add procd init script
  - script starts the daemon as user "baresip" instead of root
  - add /etc/default/baresip
  - add example configuration into /etc/baresip
  - add conffiles definition
  - prevent autostart after upgrade by using ENABLE_BARESIP variable in
    /etc/default/baresip

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>