GitLab

 - unknown message log added
 - comments fixed

Conflicts:
	ChangeLog
	src/bin/dhcp4/dhcp4_messages.mes
	src/bin/dhcp4/dhcp4_srv.cc
	src/bin/dhcp4/dhcp4_srv.h

 - missing header added
 - invalid reference to prefixes removed

Conflicts:
	src/bin/dhcp6/dhcp6_messages.mes
	src/bin/dhcp6/dhcp6_srv.cc
	src/bin/dhcp6/dhcp6_srv.h

 - pointers to DHCPv4 and DHCPv6 hooks removed
 - Many methods in HooksManager are now called directly in
   dhcp4_srv.cc, dhcp4_srv_unittest.cc and alloc_engine_unittest.cc
 - Comments wrapped up differently in dhcp4_srv_unittest.cc
 - Comments clarified in alloc_engine.{cc|h}