Wifi improvements

Contains the following 3 modifications:

* Class NetworkAccessPoint has been augmented to include macAddress and channel
* CNetworkInterfaceLinux::GetAccessPoints() adds these two new fields, and includes general readability fixups
* CNetworkInterfaceWin32::GetAccessPoints() has been implemented

topfs2 has made some progress on a network abstraction layer (https://github.com/topfs2/xbmc/tree/network_abstraction). This will probably obsolesce these changes, but in the meantime, it can't hurt to have better wifi support :)