Adding missing commentary in ncr_io.h and ncr_upd.h

Server now starts D2ClientMgr send mode after configuration if updates are
enabled, and provides an error handler to intervene if D2 communications fail.
Added new unitests in d2_unittest.h and .cc.

Added ability to suspend DDNS updates to D2ClientMgr, so updating can be
stopped when errors communications with D2 fail.

Made D2ClientMgr non-copyable to avoid programmatic errors.

Changed D2ClientMgr to return simply return from startSender() if already
in send mode.

Added enableUpdates() method to D2ClientConfig to allow DDNS updates to be
enabled or disabled independent of reconfiguration.

Changed dhcp_ddns::NameChangeSender to kick-start send process if there are
messages in the queue when startSending is called. Prior to this it required
the queuing of a new message via sendRequest().

D2ClientMgr now registers and unregisters its runReadyIO method and sender's
select-fd with IfaceMgr's external socket monitoring.

Rememebering the io_service in use and how to run ready IO handlers was
pushed down from D2ClientMgr into NameChangeSender. NameChangeSender stop
logic was altered so it will now cleanly complete the last send as well
as interrupting the daisy-chain of instigating the next send upon
completion of the current send.

d2_client.h and .cc was split into four files:

D2ClientConfig class is now in d2_client_cfg.h/cc
D2ClientMgr class is now in d2_client_mgr.h/cc

Integrates use of NameChangeUDPSender in D2ClientMgr.

Conflicts:
	ChangeLog

Conflicts:
	ChangeLog
	src/bin/dhcp4/dhcp4_srv.cc
	src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
	src/bin/dhcp4/tests/dhcp4_test_utils.h
	src/lib/dhcpsrv/cfgmgr.cc
	src/lib/dhcpsrv/cfgmgr.h

Master updated while merging in 3299.

Also ran dhcp_ddns_messages.mes through reorder tool.

This prevents errors reported by valgrind about uninitialized value of
hops being used in unit tests which skip parsing the packet. Okayed
on jabber.

b10-dhcp-ddns uses lease length as TTL in DNS update RRsets.

Shored up the error handling in the sender when the WatchSocket fails to mark
or clear. Addes unit tests accordingly.

 - getRelayInfo() implemented, relay_ is now protected
 - setRelay => setRelayInfo
 - Option::Universe is passed as const reference
 - Further clarified white_list_ field in Subnet.h

Conflicts:
	src/bin/dhcp4/tests/fqdn_unittest.cc

Fixed a couple doxgyen errors and removed the commented
out include of ioctl.h in d2_upd_unittest.cc

Made behavior of dhcp_ddns::WatchSocket a bit more robust in handling
programmatic abuse.
Cleaned up addresses used in ncr_upd_unittest.cc, added specific test
for client side address if 0.0.0.0/port 0.
Updated copyrights as appropriate and other cosmetics.

 - RelayInfoParser now takes Option::Universe as last parameter;
 - Extra tests for checking invalid parameters in RelayInfoParser
 - Comments in subnet.h improved
 - Sanity checks in RelayInfoParser implemented, doc improved

Also added a pointer to valid message types on IANA website, in the log
message description.

 - Copyright years updated
 - Minor corrections in BINDX Guide
 - dhcp4.spec relay structure description updated.
 - new unit-test for DHCPv4 client classification added
 - new unit-test for DHCPv6 client classification added
 - fixed Doxygen errors
 - comments for CfgMgr::getSubnet{4,6} updated

Changed build request methods in d2::NameAddTransaction to set the RRset TTL
value to the value of NameChangeRequest::lease_length.

Compilation under Fedora turned a couple of missing includes,
errno.h and stdint.h.