[#3531] Output fixed Pkt4 fields

[#3531] Output fixed Pkt4 fields

/src/bin/dhcp4/dhcp4_messages.mes
    Updated mesage descriptions

/src/bin/dhcp4/dhcp4_srv.cc
    Add verbose=true to Pkt4::toText() calls

/src/lib/dhcp/pkt.h
    Pkt::toText() - added verbose flag

/src/lib/dhcp/pkt4.*
    Pkt4::toText()  - added verbose logic

/src/lib/dhcp/pkt6.*
    Pkt6::toText()  - added vebose parameter (unused)

/src/lib/dhcp/tests/pkt4_unittest.cc
    TEST_F(Pkt4Test, toTextVerbose)  - new test

/src/lib/util/str.*
    printOrDump() - new function

/src/lib/util/tests/str_unittests.cc
    TEST_F(StringUtilTest, printOrDump)  - new test