drbd: Implemented receiving of new style packets on meta socket

Now drbd communication with protocol 100 actually works.
Replaced the remaining p_header80 with p_header where we
no longer know which header it is.

In the places where p_header80 is still in use, it is on
purpose, because we know that it is an old style header
there.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>