This adds set_running_config_item() which is the analogue of
get_running_config_item(). In essence it allows a caller to livepatch the
container's in-memory configuration. This POC is severly limited. Here are the
most obvious ones:
- Only the container's in-memory config can be updated but no further actions
  (e.g. on-disk actions) are made.
- Only keys in the "lxc.net." namespace can be changed. This POC also allows
  updating an existing network. For example it allows to change the network
  type of an existing network. This is obviously nonsense and in a non-POC
  implementation this should be blocked.

Use Case:
Callers can hotplug a new network for the container. For example, LXD can
create a pair of veth devices in the host and in the container and add it to
the container's in-memory config. This means, the container can later be
queried for the name of the device later on etc. Note that liblxc will
currently not delete hotplugged network devices on container shutdown since it
won't have the ifindex of the container.

Relates to https://github.com/lxc/lxd/issues/3920 .
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>