ip: improve deferred computation of checksumsHEADmain

This patch adds the same functionality for the IPv4 header checksum
as was done erlier for the SCTP/TCP/UDP transport checksum.
When the IP implementation sends a packet, it does not compute the
corresponding checksum but defers that. It will determine whether the
network interface selected for the packet has the requested capability
and computes the checksum in software, if the selected network
interface does not have the requested capability.
Do this not only for packets being sent by the local IP stack, but
also when forwarding packets. Furthermore, when such packets are
delivered to a local IP stack, do not compute or validate the checksum,
since such packets have never been on the wire. This allows to support
checksum offloading also in the case of local virtual machines or
jails. Support for epair interfaces will be added in a separate commit.

Reviewed by:		pouria, tuexen
MFC after:		1 week
Differential Revision:	https://reviews.freebsd.org/D54455

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