Modem config - use VC-MUX

I would like the config shipped with AA-supplied modems to always use VC-MUX instead of LLC encapsulation, enable bitswap and enable SRA.

* The VC-MUX option instead of LLC gives you a free extra 3.125% speed for 1500 byte IP packets.
* SRA is superb so why not go for it, although it is unfortunately not available to many users.
* [!] Bitswap is absolutely essential but is currently not set on Dlink DSL-320B-Z1 modems, which is very very bad.