mnexec already puts the shell into its own process group. Killing the entire
process group cleans up after any background processes the user left running.
The sysfs filesystem is [tagged][1] with a set of namespaces when mounted, taken
from the mounting process. Among other things, this controls which network
devices will show up in /sys/class/net and /sys/class/net/bonding_masters.
Without this change, mininet will not mount sysfs in a node. Attempting to
configure a bond interface in a node will only affect the parent namespace.
This change mounts a new sysfs filesystem in each node. To prevent this mount
from affecting the parent namespace the mount namespace is also unshared.
[1]: https://www.kernel.org/doc/Documentation/filesystems/sysfs-tagging.txt
This change uses the `Node.cmd` method instead of `Popen`. The `cmd` method
sends the input to a shell which may be in another namespace (if --innamespace
is in use), while `Popen` would always run in the root namespace.
Rich Lane