Make the kernel smarter with regards to natt_cksum_policy sysctl variable.

Now natt_cksum_policy variable controls only two behaviors:
0 - automatically handle checksums, and any other value - fully recompute
checksums. When checksums are handled automatically and if IKEd has
specified original IP addresses (i.e. checksum delta is known), checksums
will be computed incrementally. If IKEd didn't configured original
addresses, UDP checksums will be reset to zero and TCP checksums will be
ignored. When natt_cksum_policy isn't zero, checksums will be always
fully recomputed.

This allows to have NAT-T support for transport mode out of the box without
any configuration from the user side.