[5551] kea-dhcp4 now attempts to process packets with truncated VIVSO

[5551] kea-dhcp4 now attempts to process packets with truncated VIVSO

src/lib/dhcp/option.h
    SkipRemainingOptionsError - new error to signal
    that unpacking skipped options
src/lib/dhcp/option_vendor.cc
    OptionVendor::unpack() - modified to throw
    SkipRemainingOptions on truncated length

src/lib/dhcp/option_definition.cc
    OptionDefinition::optionFactory()
    Added catch-rethrow of SkipRemainginOptionsError

src/bin/dhcp4/dhcp4_messages.mes
    Added DHCP4_PACKET_OPTIONS_SKIPPED log message

src/bin/dhcp4/dhcp4_srv.cc
    Dhcpv4Srv::processPacket() - added explicit catch
    of SkipRemainingOptionsError which logs the error
    but allows the processing to continue.

src/bin/dhcp4/tests/dhcp4_srv_unittest.cc
    TEST_F(Dhcpv4SrvTest, truncatedVIVSOOption) -
    new test to verify skip-options logic for truncated
    vendor option

src/lib/dhcp/tests/pkt4_unittest.cc
    TEST_F(Pkt4Test, truncatedVendorLength) - new
    test that verifies Pkt4 unpacking of truncated VIVSO

src/lib/dhcp/tests/pkt_captures4.cc
    Pkt4Ptr PktCaptures::discoverWithValidVIVSO()
    Pkt4Ptr PktCaptures::discoverWithTruncatedVIVSO() -
    new captured discovers