Rename PQtraceSetFlags() to PQsetTraceFlags().

We have a dozen PQset*() functions.  PQresultSetInstanceData() and this
were the libpq setter functions having a different word order.  Adopt
the majority word order.

Reviewed by Alvaro Herrera and Robert Haas, though this choice of name
was not unanimous.

Discussion: https://postgr.es/m/20210605060555.GA216695@rfd.leadboat.com

src/test/modules/libpq_pipeline/libpq_pipeline.c

@@ -1326,7 +1326,7 @@ main(int argc, char **argv)
 		setvbuf(trace, NULL, PG_IOLBF, 0);
 
 		PQtrace(conn, trace);
-		PQtraceSetFlags(conn,
+		PQsetTraceFlags(conn,
 						PQTRACE_SUPPRESS_TIMESTAMPS | PQTRACE_REGRESS_MODE);
 	}