Inconsistent use of --slot/-S in pg_receivexlog and pg_recvlogical

Hi,

I noticed that pg_receivexlog is able to use --slot but not -S, even
if the code is written this way. Attached is a patch correcting that.
This makes pg_receivexlog consistent with pg_recvlogical regarding the
slot option.
IMHO, this should be backpatched to REL9_4_STABLE.
Regards,
--
Michael