[#3279] Corrected doxygen and added UTs

[#3279] Corrected doxygen and added UTs

src/bin/dhcp4/dhcp4_hooks.dox
src/bin/dhcp4/dhcp6_hooks.dox
    Correctec parameter names

src/bin/dhcp4/tests/hooks_unittest.cc
    TEST_F(HooksDhcpv4SrvTest, ddns4Update) - new test

src/bin/dhcp6/tests/hooks_unittest.cc
    TEST_F(HooksDhcpv6SrvTest, ddns6Update) - new test

src/lib/asiolink/io_service.cc
src/lib/asiolink/io_service.h
- replaced unresolvable @ref with @c for boost::asio::io_service,
  (boost code does not use doxygen)

src/lib/dns/tsigkey.h
    Replaced @class with @ref, at @class needs to point to a file