Complete the section on consensus changes from Bitcoin about zips HOT 3 CLOSED

https://github.com/zcash/zcash/wiki/Security-Auditor-Quick-Start#list-of-consensus-changes

from zips.

This section was added in 2016.0-beta-1 and extended in several versions since then. Of the list on the Security-Auditor-Quick-Start page, only these are still outstanding:

• Coinbase transactions can only be spent with a JoinSplit, providing "privacy by default."
• Checking of SIGHASH_SINGLE transactions returns proper exceptions, upstream bug not replicated.
• In non-coinbase transactions, txid computation has changed: tx input sigscripts are skipped over and JoinSplit sigs are zeroed out. Edit: this change was backed out.

Note that the following is not a consensus change, so probably should be in a ZIP rather than the spec:

• Decreased MAX_HEADERS_RESULTS (the number of block headers that are sent in a single protocol message) to 160.

from zips.

I wrote:

Checking of SIGHASH_SINGLE transactions returns proper exceptions, upstream bug not replicated.

I'm not sure this is actually a consensus change; I'll move it to a separate ticket.

from zips.