cli: allow dump to be used by any user with SELECT
cli: allow dump to be used by any user with SELECT

The previous version used a SELECT to fetch from the system.descriptor
table. This thus required SELECT on that table, which only root has. Instead
of adding some way around this like a new API call, use the existing SHOW
statements to fetch the same data (although not as elegantly).

Fixes #7964

arjun4084346/cockroach,glycerine/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,petermattis/cockroach,maxlang/cockroach,maxlang/cockroach,petermattis/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,petermattis/cockroach,arjun4084346/cockroach,maxlang/cockroach,cockroachdb/cockroach,sunya123/cockroach,paperstreet/cockroach,petermattis/cockroach,vaibhavarora/cockroach,arjun4084346/cockroach,glycerine/cockroach,paperstreet/cockroach,paperstreet/cockroach,sunya123/cockroach,glycerine/cockroach,petermattis/cockroach,sunya123/cockroach,veteranlu/cockroach,sunya123/cockroach,maxlang/cockroach,cockroachdb/cockroach,maxlang/cockroach,glycerine/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,paperstreet/cockroach,arjun4084346/cockroach,sunya123/cockroach,vivekmenezes/cockroach,maxlang/cockroach,sunya123/cockroach,petermattis/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,cockroachdb/cockroach,sunya123/cockroach,veteranlu/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,glycerine/cockroach,arjun4084346/cockroach,petermattis/cockroach,veteranlu/cockroach,arjun4084346/cockroach,glycerine/cockroach,maxlang/cockroach,petermattis/cockroach,arjun4084346/cockroach,glycerine/cockroach,arjun4084346/cockroach,glycerine/cockroach,maxlang/cockroach,vaibhavarora/cockroach,sunya123/cockroach,paperstreet/cockroach,veteranlu/cockroach,paperstreet/cockroach