This is a big-ish patch, but it's very straightforward. Under this
clang warning, we're not actually allowed to have a global variable
without a previous extern declaration for it. The cases where we
violated this rule fall into three roughly equal groups:
* Stuff that should have been static.
* Stuff that was global but where the extern was local to some
other C file.
* Stuff that was only global when built for the unit tests, that
needed a conditional extern in the headers.
The first two were IMO genuine problems; the last is a wart of how
we build tests.
In proposal 157, we added a cross-certification element for
directory authority certificates. We implemented it in
0.2.1.9-alpha. All Tor directory authorities now generate it.
Here, as planned, make it required, so that we can finally close
proposal 157.
The biggest change in the code is in the unit test data, where some
old hardcoded certs that we made long ago have become no longer
valid and now need to be replaced.
