Warns the user if the child exited with a non-zero value

Insead of writing its pid, "0" tells the system to add the pid of the
writing process to the tasks file. This fixes a bug where the actual
process was not put in the container.

Instead a being a type alias, SysFsControllerList is now a HashMap
newtype with find_path as a method. It dereferences automatically to an
immutable HashMap. It can also be constructed from a HashMap (for test
purposes).

Changed to `cgroup.procs` as it contains the whole process instead of
individual threads.

The rollback for Remove is prohibited.

Changed error messages and the order in which they are executed.
Removed the unecessary Ok(...), as the `new` should not fail.

Using a reference counter will make it easier to manage all containers:
we need to keep a non-mutable reference to the controller list for all
subcommands of the Modify command in order to find the path to the key.
A reference counter allows to have multiple references withtout
additional copies.

Useful for the GetValue command as it returns its key with the read
value.

Changed From::from to CommandResult::from as it seems clearer what type
we are casting to.

Will now use GetSetValue and FindGetSetValue.

To be used with ModifyContainer command. The path is not known during
the instantiation, so it will be determined dynamically first. If found,
the contents of the file will be stored for the rollback before writing
the new requested value.

To be used with the Exec command.
Reads and stores the contents of the file before writing to it.
Needs to know the path beforehand, as is the case with the Exec
command: the `tasks` or `cgroup.procs` file in the container folders.

Now use PathBuf::join instead of push to a mutable value

Seems clearer to me

Takes `name` as an owned string and stores it in the struct for easier
error reporting.