Rename pg_recvlogical's --create/--drop to --create-slot/--drop-slot.

A future patch (9.5 only) adds slot management to pg_receivexlog. The
verbs create/drop don't seem descriptive enough there. It seems better
to rename pg_recvlogical's commands now, in beta, than live with the
inconsistency forever.

The old form (e.g. --drop) will still be accepted by virtue of most
getopt_long() options accepting abbreviations for long commands.

Backpatch to 9.4 where pg_recvlogical was introduced.

Author: Michael Paquier and Andres Freund
Discussion: CAB7nPqQtt79U6FmhwvgqJmNyWcVCbbV-nS72j_jyPEopERg9rg@mail.gmail.com