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/interfaces/libpq/fe-trace.c

@@ -61,7 +61,7 @@ PQuntrace(PGconn *conn)
 
 /* Set flags for current tracing session */
 void
-PQtraceSetFlags(PGconn *conn, int flags)
+PQsetTraceFlags(PGconn *conn, int flags)
 {
 	if (conn == NULL)
 		return;