Create "ceph_sockaddr_storage" to serve as the portable version

On non-Linux platforms, sockaddr_storage may be larger.  Define a local
version that we use for the wire, for interoperability, and encode
from that type to/from the platform-native type.

Calculating wire length is a hack in its current version; this ought
to be cleaned up

Signed-off-by: Dan Mick <dan.mick@inktank.com>