Modularize the source files

[jserv]

The mac80211_hwsim.c in the upstream Linux kernel tree comes with more than 6000 lines in a single C source file, which sounds really terrible. However, vwifi.c has been improved a lot, resulting in a similar issue -- right now, it has 3000 lines in a single C source file. It would be great if we could split it into several functional and dedicated C files, making the source tree more modular.