system prune: remove all networks

podman system prune should also remove all networks. When we want to
users to migrate to the new network stack we recommend to run podman
system reset. However this did not remove networks and if there were
still networks around we would continue to use cni since this was
considered an old system.

There is one exception for the default network. It should not be removed
since this could cause other issues when it no longer exists. The
network backend detection logic ignores the default network so this is
fine.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

docs/source/markdown/podman-system-reset.1.md

@@ -7,7 +7,7 @@ podman\-system\-reset - Reset storage back to initial state
 **podman system reset** [*options*]
 
 ## DESCRIPTION
-**podman system reset** removes all pods, containers, images and volumes.
+**podman system reset** removes all pods, containers, images, networks and volumes.
 
 This command must be run **before** changing any of the following fields in the
 `containers.conf` or `storage.conf` files: `driver`, `static_dir`, `tmp_dir`
@@ -28,6 +28,17 @@ Print usage statement
 
 ## EXAMPLES
 
+```
+$ podman system reset
+WARNING! This will remove:
+        - all containers
+        - all pods
+        - all images
+        - all networks
+        - all build cache
+Are you sure you want to continue? [y/N] y
+```
+
 ### Switching rootless user from VFS driver to overlay with fuse-overlayfs
 
 If the user ran rootless containers without having the `fuse-overlayfs` program