[#35,!517] Added getDdnsParams() to AllocEngine::ClientContext4/6

[#35,!517] Added getDdnsParams() to AllocEngine::ClientContext4/6

src/lib/dhcpsrv/alloc_engine.*
    AllocEngine::ClientContext6::getDdnsParams()
    AllocEngine::ClientContext4::getDdnsParams() - new
    methods which return a DdnsParams instance scoped by
    currently selected subnet

    Replaced direct references to context::ddns_params_ with
    new getter methods.

src/lib/dhcpsrv/parsers/simple_parser4.cc
src/lib/dhcpsrv/parsers/simple_parser6.cc
    Removed global defaults for hostname-char-set/replacement