samples: add a mountinfo program to demonstrate statmount()/listmount()

Add a new "mountinfo" sample userland program that demonstrates how to
use statmount() and listmount() to get at the same info that
/proc/pid/mountinfo provides.

The output of the program tries to mimic the mountinfo procfile
contents. With the -p flag, it can be pointed at an arbitrary pid to
print out info about its mount namespace. With the -r flag it will
attempt to walk all of the namespaces under the pid's mount namespace
and dump out mount info from all of them.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241115-statmount-v2-1-cd29aeff9cbb@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>