epair: allow disabling receive checksum offloading

[timo.voelker_fh-muenster.de]

Summary

Allow disabling RXCSUM and RXCSUM6 on an epair interface. If disabled, epair unsets the mbuf flags that indicate a valid checksum when transferring a packet from one epair end to the other. This gives a user in a jail the power to control whether the user wants to use the result of a previous validation (by a physical interface) or not.