bind: deprecate managed-keys

This has been replaced with the "trust-anchors" keyword, per
section 8.21.1 New Features of the Bind 9 Administrator Reference
Manual:

• In order to clarify the configuration of DNSSEC keys, the trusted-keys and managed-keys statements have been deprecated, and the new trust-anchors statement should now be used for both types of key.
  When used with the keyword initial-key, trust-anchors has the same behavior as managed-keys, i.e., it configures a trust anchor that is to be maintained via RFC 5011.
  When used with the new keyword static-key, trust-anchors has the same behavior as trusted-keys, i.e., it configures a permanent trust anchor that will not automatically be updated. (This usage is not recommended for the root key.) [GL #6]

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

net/bind/files/bind/bind.keys

@@ -19,7 +19,7 @@
 # replace this file with a current version.  The latest version of
 # bind.keys can always be obtained from ISC at https://www.isc.org/bind-keys.
 
-managed-keys {
+trust-anchors {
         # ISC DLV: See https://www.isc.org/solutions/dlv for details.
         #
         # NOTE: The ISC DLV zone is being phased out as of February 2017;