Correct `ANCHOR_INPUT_WITNESS_WEIGHT` constant

Correct `ANCHOR_INPUT_WITNESS_WEIGHT` constant

`ANCHOR_INPUT_WITNESS_WEIGHT` is too high by two weight units,
likely it was calculated to include the SegWit marker bytes, but
it is used to describe an `Input::satisfaction_weight`, which does
not expect the marker bytes.

This corrects that oversight, reducing the constant by two and
adding the marker bytes back in our own internal weight
calculations. It also fixes a second issue where the constant was
too low by one when `grind_signatures` is not set, as that may
result in a signature being one byte longer than we expect.