By changing the way shell commands are defined and parsed, this makes it more
straight forward to extend the current functionality.
The readline input is parsed based on the provided command definition and the
given parameters and options are passed to a command specific callback function.
In addition, the provided command definition including its description is used
to generate a help string to display.
The help command shows a list of all supported commands or the help string for
the provided command.
Signed-off-by: Christian Ebner <c.ebner@proxmox.com>