pg_recvlogical message and code improvements.

Some error messages complained about --init and --stop being used
whereas the --create and --drop are the correct verbs. Fix that.

Also a XLogRecPtr was tested in a boolean fashion instead of being
compared to InvalidXLogRecPtr.

Backpatch to 9.4 where pg_recvlogical was introduced.

Michael Paquier
Expose -S option in pg_receivexlog.

This option is equivalent to --slot option which pg_receivexlog has
already supported, which specifies the replication slot to use for
WAL streaming. pg_recvlogical has already supported both options,
and this commit makes pg_receivexlog consistent with pg_recvlogical
regarding the slot option.

Back-patch to 9.4 where the slot option was added.

Michael Paquier
Merge branch 'master' into pg_last_xact_insert_timestamp