Fix jail examples in jib, jng, README

The provided example jail configs do not work for multiple interfaces.
Multiple interfaces need to be specified as a comma separated list or
using multiple += lines in jail.conf. In the given example, a space-
separated string is used, which doesn't work with multiple interfaces.

Also added a note to the README about VIMAGE being built-in by default
on amd64 in FreeBSD 12, with appropriate instructions for loading the
necessary netgraph ether module (ng_ether) since it is neither built-
in nor autoloads.

Submitted by:	Ryan Moeller <ryan@freqlabs.com>
Reported by:	Ryan Moeller <ryan@freqlabs.com>
MFC after:	3 days
Sponsored by:	Smule, Inc.
Differential Revision:	https://reviews.freebsd.org/D17697

share/examples/jails/README

@@ -8,6 +8,16 @@ The below 4 samples require a VIMAGE enabled kernel:
 	$ make KERNCONF=VIMAGE kernel
 	$ reboot
 
+FreeBSD 12 has VIMAGE enabled in GENERIC on amd64.
+In that case, for jng simply load the ng_ether module first
+(ng_bridge and ng_eiface will load on demand):
+
+	# (as root)
+	# Load the ng_ether module at boot:
+	$ sysrc kld_list+=ng_ether
+	# Load ng_ether at once without rebooting:
+	$ kldload ng_ether
+
 Sample 1: jail.conf(5)
 
 	$ cp jib jng /usr/sbin/

share/examples/jails/jib

@@ -51,7 +51,7 @@
 # 	#     arguments given to `jib addm xxx' in exec.prestart value.
 # 	#
 # 	vnet;
-# 	vnet.interface = "e0b_xxx e1b_xxx ...";
+# 	vnet.interface = e0b_xxx, e1b_xxx, ...;
 # 
 # 	exec.clean;
 # 	exec.system_user = "root";

share/examples/jails/jng

@@ -51,7 +51,7 @@
 # 	#     arguments given to `jng bridge xxx' in exec.prestart value.
 # 	#
 # 	vnet;
-# 	vnet.interface = "ng0_xxx ng1_xxx ...";
+# 	vnet.interface = ng0_xxx, ng1_xxx, ...;
 # 
 # 	exec.clean;
 # 	exec.system_user = "root";