[#3747] Handle multiple options for class-tagging

[#3747] Handle multiple options for class-tagging

/src/bin/dhcp4/dhcp4_srv.cc
/src/bin/dhcp6/dhcp6_srv.cc
    Replaces use of OptionDescriptor::allowedForClientClasses()
    with CfgOption::allowedForClientClasses()

/src/bin/dhcp4/tests/classify_unittest.cc
    TEST_F(ClassifyTest, classTaggingList) - new test

/src/bin/dhcp6/tests/classify_unittest.cc
    TEST_F(ClassifyTest, classTaggingList) - new test

/src/lib/dhcpsrv/cfg_option.h
    Added template function CfgOption::allowedForClientClasses()

/src/lib/dhcpsrv/tests/cfg_option_unittest.cc
    TEST_F(CfgOptionTest, allowedForClientClasses) - new test