drbd: Change how the "handshake" packets are called

Packets of type P_HAND_SHAKE define which protocol versions and features
a node supports.  For clarity, call those packets P_CONNECTION_FEATURES
instead.

(This does not determine the features that a specific drbd device
supports, such as drbd protocol A, B, C.)

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>