rv: Add rv tool

This is the (user-space) runtime verification tool, named rv.

This tool aims to be the interface for in-kernel rv monitors, as
well as the home for monitors in user-space (online asynchronous),
and in *eBPF.

The tool receives a command as the first argument, the current
commands are:

list - list all available monitors
mon - run a given monitor

Each monitor is an independent piece of software inside the
tool and can have their own arguments.

There is no monitor implemented in this patch, it only
adds the basic structure of the tool, based on rtla.

# rv --help
rv version 6.1.0-rc4: help

usage: rv command [-h] [command_options]

-h/--help: print this menu

command: run one of the following command:
list: list all available monitors
mon: run a monitor

[command options]: each command has its own set of options
run rv command -h for further information

*dot2bpf is the next patch set, depends on this, doing cleanups.

Link: https://lkml.kernel.org/r/fb51184f3b95aea0d7bfdc33ec09f4153aee84fa.1668180100.git.bristot@kernel.org

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>