Switch to 2-clause license

Approved by:	bms@
Allow drivers to specify a maximum TSO length in bytes if they are
limited in the amount of data they can handle at once.

Drivers can set ifp->if_hw_tsomax before calling ether_ifattach() to
change the limit.

The lowest allowable size is IP_MAXPACKET / 8 (8192 bytes) as anything
less wouldn't be very useful anymore.  The upper limit is still at
IP_MAXPACKET (65536 bytes).  Raising it requires further auditing of
the IPv4/v6 code path's as the length field in the IP header would
overflow leading to confusion in firewalls and others packet handler on
the real size of the packet.

The placement into "struct ifnet" is a bit hackish but the best place
that was found.  When the stack/driver boundary is updated it should
be handled in a better way.

Submitted by:	cperciva (earlier version)
Reviewed by:	cperciva
Tested by:	cperciva
MFC after:	1 week (using spare struct members to preserve ABI)
Specify a maximum TSO length limiting the segment chain to what the
Xen host side can handle after defragmentation.

This prevents the driver from throwing away too long TSO chains and
improves the performance on Amazon AWS instances with 10GigE virtual
interfaces to the normally expected throughput.

Submitted by:	cperciva (earlier version)
Reviewed by:	cperciva
Tested by:	cperciva
MFC after:	1 week
Switch to 2-clause license

Approved by:	luigi@