[#3492] Extend lenient parser check to FQDN types

[#3492] Extend lenient parser check to FQDN types

/src/lib/dhcp/libdhcp++.cc
    LibDHCP::unpackOptions4()
    LibDHCP::unpackOptions6() - split out throw so we can emit
    a more helpful log on parser errors

/src/lib/dhcp/option.h
    Added OptionParseError exception

/src/lib/dhcp/option_custom.cc
    OptionCustom::bufferLength()
    - add throw of SkipThisOptionError if lenient parsing enabled
    for OPT_FQDN_TYPE errors

/src/lib/dhcp/option_definition.cc
    OptionDefinition::factoryFqdnList()
    - add throw of SkipThisOptionError if lenient parsing enabled

/src/lib/dhcp/tests/libdhcp++_unittest.cc
    TEST_F(LibDhcpTest, unpackOptions4LenientFqdn)
    TEST_F(LibDhcpTest, unpackOptions6LenientFqdn) - new tests

/src/lib/dhcp/tests/option_custom_unittest.cc
    TEST_F(OptionCustomTest, fqdnData) - check lenient parsing behavior