Fix some compiler errors/warnings and add -p option to specify printer via the command line.

Hey Doodle3D guys,

Let me start by introducing myself, I'm Olliver and I started working for Ultimaker. I was testing some things with print3d and found a few issues and patches those in this pull request. Also I added a new command line option to the server, that allows one to chose a printer from startup rather then depending on UCI. I haven't made the UCI dependancies fully optional yet, but removing/commenting them out does make everything work without UCI. Maybe more on that later.

Thanks,

Olliver