WIP patch for Todo Item : Provide fallback_application_name in contrib/pgbench, oid2name, and dblink

This patch is to provide support for fallback application name for contrib/pgbench, oid2name, and dblink.
Currently I have done the implementation for pgbench. The implementation is same as in psql.
Before creating a final patch, I wanted to check whether my direction for creating a patch is what is expected from this Todo item.

I have done the basic testing for following 2 scenario's
1. After implementation, if during run of pgbench, we query pg_stat_activity it displays the application name as pgbench
2. It displays the application name in log file also.

Suggestions?