Include a hostname verifier rather than using the system default.

On desktop Java the system default hostname verifier doesn't do
anything. Instead HttpURLConnection does its own hostname verification
and the pluggable verifier simply supplements that.

Android's hostname verifier does everything directly.

This change copies Android's hostname verifier into OkHttp. This way
we can do hostname verification on the desktop, which wasn't possible
previously. It also brings along Android's hostname verifier tests.
Two of these tests fail on desktop Java due to differences in support
for non-ASCII subject alt names. Without our own certificate parsing
we simply live with the difference.