Use RAND_poll() for seeding randomness after fork().

OpenSSL deprecated RAND_cleanup(), and OpenSSL 1.1.0 made it into a
no-op.  Replace it with RAND_poll(), per an OpenSSL community
recommendation.  While this has no user-visible consequences under
OpenSSL defaults, it might help under non-default settings.

Daniel Gustafsson, reviewed by David Steele and Michael Paquier.

Discussion: https://postgr.es/m/9B038FA5-23E8-40D0-B932-D515E1D8F66A@yesql.se

src/backend/postmaster/fork_process.c

@@ -109,10 +109,12 @@ fork_process(void)
 		}
 
 		/*
-		 * Make sure processes do not share OpenSSL randomness state.
+		 * Make sure processes do not share OpenSSL randomness state. This is
+		 * no longer required in OpenSSL 1.1.1 and later versions, but until
+		 * we drop support for version < 1.1.1 we need to do this.
 		 */
 #ifdef USE_OPENSSL
-		RAND_cleanup();
+		RAND_poll();
 #endif
 	}