Log the cgroups version into the debug output

The migration to cgroups v2 in Fedora 31 [1] has proved a bit stormy.
This is meant to help users self-diagnose whether their problems might
be originating from the use of cgroups v2.

[1] https://fedoraproject.org/wiki/Changes/CGroupsV2

https://github.com/containers/toolbox/pull/309

toolbox

@@ -22,6 +22,7 @@ arguments=""
 assume_yes=false
 base_toolbox_command=$(basename "$0" 2>&3)
 base_toolbox_image=""
+cgroups_version=""
 
 # Based on the nameRegex value in:
 # https://github.com/containers/libpod/blob/master/libpod/options.go
@@ -442,6 +443,24 @@ enter_print_container_not_found()
 )
 
 
+get_cgroups_version()
+(
+    version=1
+
+    if ! mounts=$(mount 2>&3); then
+        echo "$base_toolbox_command: failed to detect cgroups version: couldn't list mount points" >&2
+        return 1
+    fi
+
+    if ! (echo "$mounts" | grep "^cgroup " >/dev/null 2>&3) && (echo "$mounts" | grep "^cgroup2 " >/dev/null 2>&3); then
+        version=2
+    fi
+
+    echo "$version"
+    return 0
+)
+
+
 get_group_for_sudo()
 (
     group=""
@@ -2164,6 +2183,12 @@ if [ -f /run/.containerenv ] 2>&3; then
     esac
 fi
 
+if ! cgroups_version=$(get_cgroups_version); then
+    exit 1
+fi
+
+echo "$base_toolbox_command: running on a cgroups v$cgroups_version host" >&3
+
 if [ "$op" != "reset" ] 2>&3; then
     if ! migrate; then
         exit 1