…flag

This is the beginning of a transition to using an opt-out NO_WITNESS flag
rather than an opt-in WITNESS flag. Having them temporary both at the same
time means we can do sanity checking in every transaction serialization to
be sure exactly one of both flags is set.

Most of the changes in this commit will be undone when the positive flag is
removed.

…inputs no longer possible

The base commit for segwit serialization uses extended format for transactions without inputs.
In "fixup Add segregated witness transaction serialization: revert extformat-if-empty-vin" this
was changed, but the test for CheckTransaction with no inputs was not removed. Do that now.

…rwrite and inconsistencies

…ncrementExtraNonce

…ng in miner

… update comment

…eject/punish invalid witness orphans