Mostly fixing typos and issues in comments and log messages.

 - Pkt4::check() removed
 - commented recent change in Pkt4::setType()
 - Pkt4::setType() now used OptionInt<uint8> rather than
   Option::setUint8()

Also added a few minor corrections to the DHCPv4 server and implemented
new Option6StatusCode.

1) Removed the srv-hooks logger
2) Updated DHCP4_NCR_CREATE message description
3) Updated logging doc.

Also, renamed loggers.

Except user guide related and "fixme" leftovers.

Also fixed a couple of failing unit tests.

This change comes along with the new tests for DHCPRELEASE in the
separate file, and having a separate test fixture.

Before creating NCRs the code used to check if there is a match
between a new and old lease and refused to send NCRs if there was
no match. This was wrong behavior because the allocation engine
could be reusing other client's lease in which case there could
be no match whatsoever and the NCRs should still be generated.

Also, revised the debug levels for all messages in the Dhcp4Srv class.

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::assignLease() - added logic to update the lease DB with a
    generated FQDN when not doing a fake allocation

src/bin/dhcp4/tests/fqdn_unittest.cc
    added TEST_F(NameDhcpv4SrvTest, hostnameReservation)

Fixed, copyrights, typos, minor clean up items

doc/guide/logging.xml - added new logger description
src/bin/dhcp4/dhcp4_log.cc - added bad_packet logger and log name declarations
src/bin/dhcp4/dhcp4_log.h - added bad_packet and log name externs
src/bin/dhcp4/dhcp4_messages.mes - changed drop and NAK message identifiers
src/bin/dhcp4/dhcp4_srv.cc - changed packet drop and NAK log invocations
src/bin/dhcp4/main.cc - used extern for log name

Log the ciaddr and requested-ip-address if exists. Previously the message
said it logged yiaddr sent by the client, but the client doesn't set the
yiaddr.