Server mode command for just preprocessing to get files list

[sim642]

In #606 a command was added to get the list of involved files in the analysis. However, currently this is only available in the server after doing a complete analysis.

For goblint/GobPie#17 and goblint/GobPie#31, it would be useful to get the list of files already on startup (so GobPie knows which files should even trigger analysis and for which server instance) without having to wait for the entire analysis to complete.
This would be possible because getting the list of files only requires preprocessing and parsing (not even merging),