fix `update-rc.d` compatibility by resolving script name via readlink

`update-rc.d` creates symlinks of this script named like `/etc/rc3.d/S20tt-rss` and it fails on boot because tries to use defaults from non-existent `/etc/default/S20tt-rss`
Merge pull request #1 from YuppY/patch-1

fix `update-rc.d` compatibility by resolving script name via readlink