[#4142] Initial fix minus UTs

[#4142] Initial fix minus UTs

/src/bin/dhcp4/dhcp4_messages.*
    DHCP4_CLIENT_HOSTNAME_SCRUBBED_EMPTY
    DHCP4_CLIENT_FQDN_SCRUBBED_EMPTY - new messages

/src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::processClientFqdnOption() - catch FQDNScrubbedEmtpy, log and drop FQDN option

    Dhcpv4Srv::processHostnameOption() - log and drop hostname option if scrubbed empty

/src/bin/dhcp6/dhcp6_messages.*
    DHCP6_CLIENT_FQDN_SCRUBBED_EMPTY - new message

/src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::processClientFqdn() - catch FQDNScrubbedEmtpy, log and drop FQDN option

/src/lib/dhcpsrv/d2_client_mgr.h
    FQDNScrubbedEmpty - new exception type
    D2ClientMgr:: adjustDomainName() - throw FQDNScrubbedEmpty when FQDN is scrubbed empty