Add support for updating restart policy

This is something Docker does, and we did not do until now. Most
difficult/annoying part was the REST API, where I did not really
want to modify the struct being sent, so I made the new restart
policy parameters query parameters instead.

Testing was also a bit annoying, because testing restart policy
always is.

Signed-off-by: Matt Heon <mheon@redhat.com>

libpod/define/container.go

@@ -1,5 +1,9 @@
 package define
 
+import (
+	"fmt"
+)
+
 // Valid restart policy types.
 const (
 	// RestartPolicyNone indicates that no restart policy has been requested
@@ -27,6 +31,16 @@ var RestartPolicyMap = map[string]string{
 	RestartPolicyUnlessStopped: RestartPolicyUnlessStopped,
 }
 
+// Validate that the given string is a valid restart policy.
+func ValidateRestartPolicy(policy string) error {
+	switch policy {
+	case RestartPolicyNone, RestartPolicyNo, RestartPolicyOnFailure, RestartPolicyAlways, RestartPolicyUnlessStopped:
+		return nil
+	default:
+		return fmt.Errorf("%q is not a valid restart policy: %w", policy, ErrInvalidArg)
+	}
+}
+
 // InitContainerTypes
 const (
 	// AlwaysInitContainer is an init container that runs on each