Provide dns Message and EDNSMessage constructors which accept FlagConstants and ValueConstants

[rwall]

In ticket:5675#comment:16 exarkun wrote:

1. Earlier you mentioned an idea about improving the handling of these numerous flags. I think that's a good idea to pursue. Consider this idea (not necessarily for this ticket, since making these APIs private means we have plenty of time to improve them with follow-up work) - __init__ should accept a FlagConstant collapsing as many of these flags as possible (looks like there are 8 parameters that would all combine into 1) into a single value. We could also think about whether opCode, rCode, and ednsVersion merit more special handling (perhaps a Values constant instead of int) and For compatibility, _EDNSMessage can also:

1. provide an alternate constructor with a signature more like its current __init__ signature - this would be an API more like a drop-in replacement for Message.__init__. This can also include whatever extra safety checks seem useful to help developers avoid subtle mistakes (though hopefully we can also encourage them to switch to the nicer API).