ifp->if_capabilities needs to grow

Hi,

As part of ongoing TLS RX work, it has become apparent that 
if_capabilities and if_capenable needs to grow to 64-bits at least.

Right now those fields are 32-bits, and are fully utilized.

The question is how this can be implemented so that a MFC to 13-stable 
is possible.

The most simple solution is to substitute "int" by "uint64_t", but that 
will break the ABI.

Another solution is to use an array of "int" variables.

A third solution is to abandon the two fields when MFC-ing, and adding 
two new 64-bit fields in the end of the ifnet.

Also the user-space API for ifconfig is subject to change.

--HPS