engine: resources: Remove named return params

Named return params aren't a favourite feature of mine, and they're
rarely used in the resource writing. They keep popping up because I once
used them and we've been copying and pasting code ever since. Get rid of
them all to help prevent the unnecessary spread.

engine/resources/nspawn.go

@@ -201,7 +201,7 @@ func (obj *NspawnRes) Watch() error {
 // CheckApply is run to check the state and, if apply is true, to apply the
 // necessary changes to reach the desired state. This is run before Watch and
 // again if Watch finds a change occurring to the state.
-func (obj *NspawnRes) CheckApply(apply bool) (checkOK bool, err error) {
+func (obj *NspawnRes) CheckApply(apply bool) (bool, error) {
 	// this resource depends on systemd to ensure that it's running
 	if !systemdUtil.IsRunningSystemd() {
 		return false, errors.New("systemd is not running")