RFC: TSO patch for current

This is a patch for the stack and the em driver to enable TSO
on CURRENT. Previously I had problems getting it to work, but
this is functional.

I should note that CURRENT is being a pain right now, when
I comment out em in the config the kernel panics coming up,
so I had to substitute this code into the tree. Rather bizarre :)

I have this functionality running on a 6.1 based system, and
our test group is already testing against that driver, so far
things are looking good.

I have designed it so the driver can continue to be built
without support. There is also a sysctl in the stack code
so you can set net.inet.tcp.tso_enable on or off and
compare.

I know there may be some refinements to add in, but I
would like to get this into CURRENT as a start.

Comments?

Jack