Reduce flickering by adding pixmap for each monitor

Turns out the window was cleaned, then the offscreen pixmap was
populated, then the pixmap was copied to the window, which increased the
perceived flickering.

Now we wait with cleaning the window until the offscreen pixmap is
populated, then clean the window, then copy the pixmap on the window.
This requires an additional pixmap the size of each window (it was the
most simple solution that wouldn't require me to recalculate the
position offsets and sizes and stuff).
Fix allocation sizes for icon/module/monitor lists

The list contains *pointers* to those structures, so we needn't actually
allocate memory for entire structures.
Add network module

It displays the name of each network interface, next to its IP address.

Currently it hasn't got any icons, and (theoretically) it also displays
interfaces that are down but still have an address associated.
[wm] Use errno and fix return value handling for socket()