Make pg_receivexlog silent with 9.3 and older servers

A pointless and confusing error message is shown to the user when
attempting to identify a 9.3 or older remote server with a 9.5/9.6
pg_receivexlog, because the return signature of IDENTIFY_SYSTEM was
changed in 9.4.  There's no good reason for the warning message, so
shuffle code around to keep it quiet.

(pg_recvlogical is also affected by this commit, but since it obviously
cannot work with 9.3 that doesn't actually matter much.)

Backpatch to 9.5.

Reported by Marco Nenciarini, who also wrote the initial patch.  Further
tweaked by Robert Haas and Fujii Masao; reviewed by Michael Paquier and
Craig Ringer.