Use PqMsg_* macros in postgres.c.

Commit f4b54e1ed9, which introduced macros for protocol characters,
missed updating a couple of places in postgres.c.

Author: Dave Cramer
Reviewed-by: Fabrízio de Royes Mello
Discussion: https://postgr.es/m/CADK3HHJUVBPoVOmFesPB-fN8_dYt%2BQELV2UB6jxOW2Z40qF-qw%40mail.gmail.com
Backpatch-through: 17

src/backend/tcop/postgres.c

@@ -323,7 +323,7 @@ InteractiveBackend(StringInfo inBuf)
 		printf("statement: %s\n", inBuf->data);
 	fflush(stdout);
 
-	return 'Q';
+	return PqMsg_Query;
 }
 
 /*
@@ -4966,9 +4966,9 @@ PostgresMain(const char *dbname, const char *username)
 				break;
 
 				/*
-				 * 'X' means that the frontend is closing down the socket. EOF
-				 * means unexpected loss of frontend connection. Either way,
-				 * perform normal shutdown.
+				 * PqMsg_Terminate means that the frontend is closing down the
+				 * socket. EOF means unexpected loss of frontend connection.
+				 * Either way, perform normal shutdown.
 				 */
 			case EOF: