Apply a slightly modified version of a patch provided by Pierre Neyron
that allows to enable/disable new network interfaces by default.
This feature is especially helpful for VM or container setups where many
network interfaces are created.
- Adding a sensor in the mbmon support stored the added sensor in a
pointer but never used it afterwards
- Drop pixbuf width in draw_left_frame_overlap, the code unlike other
drawing helpers does not use the pixbuf width
- Remove unused scroll selection from net_stats_page
This fixes several set but unread warnings when compiling with a recent
gcc (encountered with gcc 4.8.2).