Update protobuf library to v2

There is a new protobuf library (and corresponding code generator) for
Go: google.golang.org/protobuf.

It is fairly compatible with the previous v1 API
(github.com/golang/protobuf), but there are some changes.

This patch adjusts the code and generated files to the new API.

The on-wire/on-disk format remains unchanged so this should be
transparent to the users.

cmd/chasquid-util/test.sh

@@ -74,7 +74,7 @@ if [ "$A" != "(email)  user@somewhere" ]; then
 fi
 
 C=$(r print-config | grep hostname)
-if [ "$C" != "hostname: \"$HOSTNAME\"" ]; then
+if ! ( echo "$C" | grep -E -q "hostname:.*\"$HOSTNAME\"" ); then
 	echo print-config failed
 	echo output: "$C"
 	exit 1