Currently the loopback interface does not work with SCTP/TCP/UDP if IFCAP_RXCSUM is disabled and IFCAP_TXCSUM is enabled. This patch fixes that and cleans up checksum offloading-related stuff.

• Allow disabling IFCAP_RXCSUM_IPV6 or IFCAP_TXCSUM_IPV6.
• Do not pretend the checksum is correct by setting the LO_CSUM_SET flags if IFCAP_RXCSUM_IPV6 or IFCAP_RXCSUM is enabled. Instead, remove the LO_CSUM_SET flags (in case they have been set somehow) if IFCAP_RXCSUM_IPV6 or IFCAP_RXCSUM is disabled.
• Do not unset the transmit checksum offload flags LO_CSUM_FEATURES or LO_CSUM_FEATURES6 since they have a meaning for the receive path since D51475 and D54455.

Initially, ssh 127.0.0.1 works. After disabling IFCAP_RXCSUM,

ifconfig lo0 -rxcsum

ssh 127.0.0.1 does not work. It works after this patch has been applied.