if_tuntap: support receive checksum offloading for tap interfaces

When enabled, pretend that the IPv4 and transport layer checksum
is correct for packets injected via the character device.
This is a prerequisite for adding support for LRO, which will
be added next. Then packetdrill can be used to test the LRO
code in local mode.

Reviewed by: rscheff, zlei
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D42477