Added commentary, updated sub-options unpacking,
    and added support for skip logic to kea-dhcp6.

src/lib/dhcp/option.h
    Added detail to SkipRemainingOptionsError commentary

src/lib/dhcp/libdhcp++.cc
    LibDHCP::unpackVendorOptions4()
    LibDHCP::unpackVendorOptions6()

src/lib/dhcp/tests/pkt4_unittest.cc
    TEST_F(Pkt4Test, unpackVendorMalformed) - updated for
    new exception type

    TEST_F(Pkt4Test, truncatedVendorLength) - removed
    extraneous byte streams

src/lib/dhcp/tests/pkt6_unittest.cc
    TEST_F(Pkt6Test, unpackVendorMalformed) - updated for
    new exception type

    TEST_F(Pkt6Test, truncatedVendorLength) - new test for
    truncated vendor options

src/lib/dhcp/tests/pkt_captures.h
src/lib/dhcp/tests/pkt_captures.cc
    isc::dhcp::Pkt6Ptr captureSolicitWithVIVSO()
    isc::dhcp::Pkt6Ptr captureSolicitWithTruncatedVIVSO() -
    new captured packets

src/bin/dhcp6/dhcp6_messages.mes
    DHCP6_PACKET_OPTIONS_SKIPPED - new log message

src/bin/dhcp6/dhcp6_srv.cc
    Dhcpv6Srv::processPacket() - added explicit catch of
    SkipRemainingOptionsError to allow processing the packet

src/bin/dhcp6/tests/dhcp6_srv_unittest.cc
    TEST_F(Dhcpv6SrvTest, truncatedVIVSO) - new test to verify
    server's ability to handle SkipRemainingOptionsError