Implement transmit checksum offload.

The basic strategy is to put the pseudo hdr sum into the packet
at the TCP/UDP layer and assume we can do checksum offload.
Then when we know we cannot do checksum offload (ip frags,
sending on ether medium to a device which cannot offload,
or sending on a non-loopback medium), we complete the full
checksum.

I have only implemented hardware checksum offload on
an out-of-tree device.

Reviewed-by: rminnich@gmail.com
Signed-off-by: Andrew Gallatin <gallatin@google.com>
Allow hardware devices to pad frames to mintu

Rather than reallocating & copying frames to pad them
to 60 bytes (ethernet mintu), introduce a new feature NETF_PADMIN,
which a device can declare if it is capable of padding frames to
the min mtu.  If a device is not capable, we pad the frame just before
handing it to the device.

Reviewed-by: rminnich@gmail.com
Signed-off-by: Andrew Gallatin <gallatin@google.com>