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
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D42477

(cherry picked from commit ff69d13a50d1d07601de0885fd94f6a09a7ba383)