mountd/exportfs: Make m_addrlist field a nfs_sockaddr

mountd/exportfs: Make m_addrlist field a nfs_sockaddr

To store non-AF_INET addresses in the nfs_client structure, we need to
use more than in_addr for the m_addrlist field.  Make m_addrlist
larger, then add a few helper functions to handle type casting and
array indexing cleanly.

We could treat the nfs_client address list as if all the addresses
in the list were the same family.  This might work for MCL_SUBNETWORK
type nfs_clients.  However, during the transition to IPv6, most hosts
will have at least one IPv4 and one IPv6 address.  For MCL_FQDN, I
think we need to have the ability to store addresses from both
families in one nfs_client.

Additionally, IPv6 scope IDs are not part of struct sin6_addr.  To
support link-local IPv6 addresses and the like, a scope ID must be
stored.

Thus, each slot in the address list needs to be capable of storing an
entire socket address, and not simply the network address part.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>