for now, only mix in sysctl hw.{uuid,serialno,sensors} to /dev/random.

as found the hard way by d.rauschenb@gmail on an old fujitsu siemens
machine, reading all of hw (notable hw.setperf) can have unexpected
side-effects.  ok deraadt

src/etc/rc

@@ -1,4 +1,4 @@
-#	$OpenBSD: rc,v 1.538 2019/10/02 17:15:40 sthen Exp $
+#	$OpenBSD: rc,v 1.539 2019/10/06 16:16:19 sthen Exp $
 
 # System startup script run by init on autoboot or after single-user.
 # Output and error are redirected to console by init, and the console is the
@@ -445,7 +445,7 @@ sh /etc/netstart
 
 # Any write triggers a rekey.
 dmesg >/dev/random
-sysctl hw >/dev/random
+sysctl hw.{uuid,serialno,sensors} >/dev/random 2>&1
 
 # Load pf rules and bring up pfsync interface.
 if [[ $pf != NO ]]; then