tkey.c:buildquery() error path cleanup code is incomplete

In main, tkey.c:buildquery() fails to cleanup rdata (by calling dns_message_puttemprdata()) in its error path.

In v9_16 and v9_18 it also fails to cleanup tkeylist and tkeyset, although the error paths that could have required the cleanup of those variables never take place (those RETERRs never jump, because the functions that are being called currently always return success). In main those functions were refactored to return void, so most RETERRs are gone.

!6652 (merged) further refactors buildquery() for main, so that RETERRs and the catchall error path will not be needed.

Note: the buildquery() function is currently used (indirectly) only by the nsupdate program.