sql: disallow returning many rows from subselect

To follow ANSI SQL standard we should dissallow returning
multiple rows from subselects after = and in braces ().
To achieve this goal we have introduced Select SF_SingleRow
flag that indicates the case of subselect having no
client-defined LIMIT 1 to patch system implicit LIMIT 1 to
be LIMIT 2 and generate extra bytecode to HALT execution on
reaching this restrict.

Resolves #2366.