pg_recvlogical fixes

Hi,

While testing wal2json, I faced some problems with pg_recvlogical.
Attached is a serie of patches that can improve pg_recvlogical. Patches
#2 and #3 are bugfixes (and should be applied to 9.5 too). Patch #1 is
not mandatory to 9.5.

Short description:

#1: add a bunch of checks to complain when using an option that is not
available in the specified action;
#2: there is a wrong check because startpos option can be specified with
--create-slot;
#3: doesn't ignore startpos in --create-slot because that action could
be specified together with --start action (that uses that option);

--
Euler Taveira Timbira - http://www.timbira.com.br/
PostgreSQL: Consultoria, Desenvolvimento, Suporte 24x7 e Treinamento