Allow specifying default flag values in just one place

[arikkfir]

Currently, to specify a default value for a flag, it has to have the same default value in all places it is mentioned. This is not great when the same flag is referenced from multiple places - e.g. in the action as well as in one or more hooks.

Consider:

• Only taking default values when they are non-zero
• Default values specified in the action always override those specified in hooks (if any)