pg_regress: Promptly detect failed postmaster startup.

Detect it the way pg_ctl's wait_for_postmaster() does.  When pg_regress
spawned a postmaster that failed startup, we were detecting that only
with "pg_regress: postmaster did not respond within 60 seconds".
Back-patch to 9.4 (all supported versions).

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/20181231172922.GA199150@gust.leadboat.com

src/test/regress/pg_regress.c

@@ -2419,7 +2419,7 @@ regression_main(int argc, char *argv[], init_function ifunc, test_function tfunc
 			 * Fail immediately if postmaster has exited
 			 */
 #ifndef WIN32
-			if (kill(postmaster_pid, 0) != 0)
+			if (waitpid(postmaster_pid, NULL, WNOHANG) == postmaster_pid)
 #else
 			if (WaitForSingleObject(postmaster_pid, 0) == WAIT_OBJECT_0)
 #endif