We now always write the "optional" streaming data descriptor
signature, which turns out to be required for OS X.

Also, handle reading the data descriptor with or without the
signature, per the spec's recommendation. Fix data descriptor
reading bugs found in the process.

Fixes #3252

R=golang-dev, alex.brainman, nigeltao, rsc
CC=golang-dev
https://golang.org/cl/5787062