Refine cli interface

[importantchoice]

The are some inconsistencies in the cli interface.

• The metadata subcommand does not require a device (-d), but all other commands do.
• The info option of fileslack subcommand should not require a metadata file

Also it might be nicer to move the -d option behind the subcommand.

The argparse configuration should require all options which are actually required and should not require options, which are not required...

Edit:

• Maybe we should seperate informational functionality (fattools, metadata) from hiding techniques via additional subgroups
• Read/write/clear/info options of a hiding technique should only be used once at a time. We could propably implement this via add_mutually_exclusive_group
• for fileslack subcommand, the -d option must be required, when writing to fileslack

Some other things are wrong or need extension in the help output:

• as the mftslack options were copied from fileslack, this keyword occures in the help output, but is wrong there.
• subcommand help output should be more descriptive