netlabel: Add an address family to domain hash entries.

The reason is to allow different labelling protocols for
different address families with the same domain.

This requires the addition of an address family attribute
in the netlink communication protocol.  It is used in several
messages:

NLBL_MGMT_C_ADD and NLBL_MGMT_C_ADDDEF take it as an optional
attribute for the unlabelled protocol.  It may be one of AF_INET,
AF_INET6 or AF_UNSPEC (to specify both address families).  If it
is missing, it defaults to AF_UNSPEC.

NLBL_MGMT_C_LISTALL and NLBL_MGMT_C_LISTDEF return it as part of
the enumeration of each item.  Addtionally, it may be sent to
LISTDEF to specify which address family to return.

Signed-off-by: Huw Davies <huw@codeweavers.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>