This commit teaches the SSH server to recognize the "upgrade"
request line that clients send when they wish to switch the
channel to version 2 of the SSH protocol.
Servers don't honor upgrade requests unless an experimental config
option is set.
Since the built-in server now supports upgrade requests, our test
server to test the handshake has been deleted. Existing tests
use the built-in server and their output doesn't change.
The upgrade is handled in our state machine. The end result is a bit
wonky, as the server transitions back to version 1 state immediately
after upgrading. But this will change as soon as version 2 has an
actual protocol that differs from version 1.
Tests demonstrating that the new server is a bit more strict about
the upgrade handshake have been added.