Including replication slot data in base backups

Hi all,

As of now, pg_basebackup creates an empty repository for pg_replslot/
in a base backup, forcing the user to recreate slots on other nodes of
the cluster with pg_create_*_replication_slot, or copy pg_replslot
from another node. This is not really user-friendly especially after a
failover where a given slave may not have the replication slot
information of the master that it is replacing.

The simple patch attached adds a new option in pg_basebackup, called
--replication-slot, allowing to include replication slot information
in a base backup. This is done by extending the command BASE_BACKUP in
the replication protocol.

As it is too late for 9.4, I would like to add it to the first commit
fest of 9.5. Comments are welcome.

Regards,
--
Michael