vtnet: improve interface capability handling

Enable the handling of the IFCAP_RXCSUM_IPV6 handling by handling
IFCAP_RXCSUM and IFCAP_RXCSUM_IPV6 as a pair. Also make clear, that
software and hardware LRO require receive checksum offload.

Reviewed by: Timo Völker
Differential Revision: https://reviews.freebsd.org/D52682

(cherry picked from commit eaf619fddcb21859311b895a0836da3171a01531)