wifi_statistics: Add support for multiple monitor interfaces

This requires changing the general concept how interfaces are activated.
Instead of accepting module parameters, add an "activate" file which
enables or disables statistics for the corresponding monitor interface.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
wifi_statistics: change output format to JSON

The previous output format was more a mess than anything.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>