Add commented-out examples for net.inet.tcp.always_keepalive / keepidle.

These are useful if you're behind a bad nat with short timeouts as often
found in airport lounges and hotels. If the keepalives fail because the
network goes away (moving out of wireless reception, swapping cables etc)
your connections are more likely to drop so this is a double-edged sword.
ok henning@

src/etc/sysctl.conf

@@ -1,4 +1,4 @@
-#	$OpenBSD: sysctl.conf,v 1.52 2011/06/24 19:47:48 naddy Exp $
+#	$OpenBSD: sysctl.conf,v 1.53 2012/05/31 15:04:03 sthen Exp $
 #
 # This file contains a list of sysctl options the user wants set at
 # boot time.  See sysctl(3) and sysctl(8) for more information on
@@ -13,6 +13,8 @@
 #net.inet6.ip6.mforwarding=1	# 1=Permit forwarding (routing) of IPv6 multicast packets
 #net.inet6.ip6.multipath=1	# 1=Enable IPv6 multipath routing
 #net.inet6.ip6.accept_rtadv=1	# 1=Permit IPv6 autoconf (forwarding must be 0)
+#net.inet.tcp.always_keepalive=1 # 1=Keepalives for all connections (e.g. hotel/airport NAT)
+#net.inet.tcp.keepidle=100	# 100=send TCP keepalives every 50 seconds
 #net.inet.tcp.rfc1323=0		# 0=Disable TCP RFC1323 extensions (for if tcp is slow)
 #net.inet.tcp.rfc3390=0		# 0=Disable RFC3390 for TCP window increasing
 #net.inet.esp.enable=0		# 0=Disable the ESP IPsec protocol