Fix wrong formatting of 0.0.0.0/X table records in ipfw(8).

Add `flags` u16 field to the hole in ipfw_table_xentry structure.
Kernel has been guessing address family for supplied record based
on xent length size.
Userland, however, has been getting fixed-size ipfw_table_xentry structures
guessing address family by checking address by IN6_IS_ADDR_V4COMPAT().

Fix this behavior by providing specific IPFW_TCF_INET flag for IPv4 records.