devfs: add bpf example

Add an example for allowing members of the network group to read from
bpf devices. In particular, this allows members of the network group
to monitor traffic without running with root privileges.

Reviewed by:		markj, glebius
Sponsored by:		Netflix, Inc.
Differential Revision:	https://reviews.freebsd.org/D56742

sbin/devfs/devfs.conf

@@ -40,3 +40,9 @@
 # Allow members of group operator to cat things to the speaker
 #own	speaker	root:operator
 #perm	speaker	0660
+
+# Allow members of group network to read from bpf devices.
+# In particular, this allows all group members to capture all
+# network traffic using tcpdump or wireshark.
+#own	bpf	root:network
+#perm	bpf	0640