Not exposed APIReadyCheckFunc to outside of package

APIReadyCheckFunc is used only in api, hence it can be decapitalize
to make its scope only in this package. This fix changes its scope.
In addition, api.APIReadyCheckFunc seems to be redundant so the name
is changed. Change the comment to fit to golang style, too.

pkg/server/api/shim.go

@@ -40,8 +40,8 @@ type ShimNetConf struct {
 	LogToStderr     bool   `json:"logToStderr,omitempty"`
 }
 
-// Define a type for API readiness check functions
-type APIReadyCheckFunc func(string) error
+// readyCheckFunc defines a type for API readiness check functions
+type readyCheckFunc func(string) error
 
 // CmdAdd implements the CNI spec ADD command handler
 func CmdAdd(args *skel.CmdArgs) error {
@@ -74,7 +74,7 @@ func CmdDel(args *skel.CmdArgs) error {
 	return nil
 }
 
-func postRequest(args *skel.CmdArgs, readinessCheck APIReadyCheckFunc) (*Response, string, error) {
+func postRequest(args *skel.CmdArgs, readinessCheck readyCheckFunc) (*Response, string, error) {
 	multusShimConfig, err := shimConfig(args.StdinData)
 	if err != nil {
 		return nil, "", fmt.Errorf("invalid CNI configuration passed to multus-shim: %w", err)