cubicweb #3836889 ambiguous calls to eschema.rdef() [in-progress]

in a few places we call eschema.rdef() where multiple rdefs are possible. yams warns. we should investigate each call site and check if it needs a loop over rdefs or a different fix.

done in<not specified>
closed by<not specified>
patch[entity/rset] replace scary warnings about ambiguous rdefs by XXX in the code [applied]