edit: add `--force` option

This is usefull if the unit files or mount point have been given away
for some reasons.

refactorization command declaration

I'm tired of having argparser declarations at the botton of the module
and the command core function above.

It's quite crappy, but, well, it's easier to write. Maybe, later, we'd
like to remove this stuff, but for now it seems ok as we do not need
complex argparse configuration.

status: add status command

Simply call the `systemctl status` command for both automount and mount units.

show: small fixes

show: add command to display useful information about the layer

Having an easy way to retrieve the unit names is useful when trying to manually start them.

To debug the layer, it is useful to display the mount command.

reset: add command

This command is really usefull when we are using containers to tests things.

update README

normalize code

README: add a readme file with typical use case.

edit: fix, propagate changes to descendants

interrup/preserve: refactorize and rename options

dump: prevent writting info file on errors

remove dependency to `sh` package as it is nomore used

edit: stop/restart descendannts automatically with --stop-others.

editer: user is the boss

In a few situation, OverlayFS is ok with that.
If the user insists, let's do it.

edit: prevent broken mount point

Do not allow users to edit layers having active descendants.

rewrite code to make it a bit more easy to improve and add tests

Tests for list are still missing. But it's late.

edit: restart units only if they were started before

coding style

edit: ignore fully empty file