Hide other user's pg_stat_ssl rows

Hide other user's pg_stat_ssl rows

Change pg_stat_ssl so that an unprivileged user can only see their own
rows; other rows will be all null.  This makes the behavior consistent
with pg_stat_activity, where information about where the connection
came from is also restricted.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/63117976-d02c-c8e2-3aef-caa31a5ab8d3%402ndquadrant.com