Pre-/post- options in fuse to implement a post-process

Instead of resolving #3538 (that is !-hard), @flothoni proposed a solution with server-fuse to implement a post-process and still to be admin-configurable by configs.

A post-process programm is something that takes a .vidjil file and give back another .vidjil file.

We implement two --pre and --post options to server-fuse

• --pre progname will lauch progname in.vidjil in2.vidjil on each in.vidjil input file.
• --post progname will lauch progname out.vidjil out2.vidjil on the collated out.vidjil file