Implement proper branches

Implement proper branches

Trying to implement differing negotiation based on parameters that just
happen to be there is a bad strategy, as this bug demonstrates.

Instead, add a parameter to the negotiate() function that is a bitmask
telling us which part of the negotiation we would like to see: modern vs
oldstyle, initial phase versus 'the rest'.

This doesn't actually survive 'make check' yet, because of a different
bit of crack that's still in there. Different bug, different commit.