app.{Common,demux}: make gzip options global

While starting to port other sub-commands to the common options it
became obvious that passing all those arguments was not scalable. The
`Global_configuration` module contains *global* references but at least
provides and unified API.
app: move I/O buffer-sizes to Global_configuration
app.random: homogenize command line (App_common)
Common: fix function file_to_file (use config)
Common: improve Command_line.gzip_output_flags
app.transform: use Command_line.gzip_output_flags
app.random: add verbosity flags
app.Common: add code documentation