yams #142386 Clarify RelationType vs. RelationDefinition [open]

There are two options :

• either use RType attributes that are meaningful for RDefs as a "default" value for new RDefs
• or completely deny setting attributes that are meant to be set on RDefs (like __permissions__) on RTypes.

After some discussions at Logilab (syt, auc, fcayre, pyves), the second choice was preferred.

However, the present facility offered to generate RTypes implicitely from a RDef is to be kept.