Add incidents_dir config field

On the [node] section of the tahoe.cfg now there is a field 'incidents_dir'
where the path to the incidents foder can be configured.
return default introducer web.port to none

No web ui for introducers has been the default behavior for a long time, so
lets leave it that way now even though create-introducer can configure a
web.port now. This also makes the one test that was failing pass.
Add --logfile option to start and restart

Make it possible to specify the file where the logs will be written.
Add --pidfile option

Used to tell tahoe the file where the pid of the twisted process will be
stored.
Add an option to specify tempdir upon node creation.