Define and use two macros for loopback checksum offload:

Define and use two macros for loopback checksum offload:

LO_CSUM_FEATURES - a bitmask of supported transmit offload features, which
  will be stored in if_hwassist if IFCAP_TXCSUM is enabled, and be cleared
  from mbuf packet header csum flags on transmit. (1)

LO_CSUM_SET - a bitmask of supported receive offload features, which will
  be set on the mbuf packet header csum flags on transmit if IFCAP_RXCSUM
  is enabled.

While here, fix SCTP offload for loopback: offer generation on the
transmit side, don't just skip validation on the receive side.

Obtained from:  DragonflyBSD (1)
MFC after:      1 week