Unconditionally enable support for O_IPSEC opcode.

IPsec support can be loaded as kernel module, thus do not depend from
kernel option IPSEC and always build O_IPSEC opcode implementation as
enabled.

Do not invoke IPv4 NAT handler for non IPv4 packets. Libalias expects
a packet is IPv4. And in case when it is IPv6, it just translates them
as IPv4. This leads to corruption and in some cases to panics.
In particular a panic can happen when value of ip6_plen modified to
something that leads to IP fragmentation, but actual packet length does
not match the IP length.

Packets that are not IPv4 will be dropped by NAT rule.