Multiple screen support in linuxfb

The counterpart to QTBUG-36113.

Like eglfs, linuxfb can only use one framebuffer device at the moment. The good news is that, unlike eglfs, the device is at least configurable on the command-line.

However, there is no reason to restrict ourselves to exposing only one platform screen (and so one QScreen). The plugin should be changed to support multiple fb=... and related parameters and create and maintain a screen for each.