Check for existing replication slot in pg_createsubscriber

Hackers,

I noticed in testing and usage that pg_createsubscriber doesn't check for
an existing replication slot before attempting to create one, whereas it
*does* check for existing publications.
If it were to check for an existing replication slot, then the --dry-run
mode would be able to detect the issue.

If this seems like a good feature, I'm happy to try and put together a
patch for it.

Thanks,
Zane