Incorrect log message and checks in pgrecvlogical

Hi,

While looking at pg_recvlogical code, I noticed that a couple of error
messages are incorrect when invoking some combinations of --create,
--start or --drop.

For example, here --init should be --create, --stop should be --drop:
$ pg_recvlogical --create --drop --slot foo
pg_recvlogical: cannot use --init or --start together with --stop
Try "pg_recvlogical --help" for more information.

Also, when checking the combination startpos && (create || drop), I
think that we should check that startpos is InvalidXLogRecPtr.

The patch attached fixes all those things. It should be back-patched to 9.4.
Regards,
--
Michael