Moving the parameters needs to be done before defaults are applied to the
config, so moveDdnsParams was changed to modify a mutable top level
element map instead of SrvConfig contents.

src/lib/dhcpsrv/parsers/simple_parser4.cc
src/lib/dhcpsrv/parsers/simple_parser6.cc
    Change ddns-send-updates default to true.

src/lib/dhcpsrv/srv_config.*
    SrvConfig::getConfiguredGlobal() - new method to fetch a
    global by name

    SrvConfig::moveDdnsParams() - changed to accept/modify
    a top-level Element map

src/lib/dhcpsrv/tests/srv_config_unittest.cc
    updated unit tests accordingly