When requesting a particular replication slot, the new pg_basebackup
option -C/--create-slot creates it before starting to replicate from it.

Further refactor the slot creation logic to include the temporary slot
creation logic into the same function.  Add new arguments is_temporary
and preserve_wal to CreateReplicationSlot().  Print in --verbose mode
that a slot has been created.

Author: Michael Banck <michael.banck@credativ.de>