make pg_createsubscriber option names more consistent

I propose to rename the pg_createsubscriber option --socket-directory to 
--socketdir.  This would make it match the equivalent option in 
pg_upgrade.  (It even has the same short option '-s'.) 
pg_createsubscriber and pg_upgrade have a lot of common terminology and 
a similar operating mode, so it would make sense to keep this consistent.