branch-merge: land udev support

A certain class of machines don't work in plymouth because
they draw the kernel console to /dev/dri/card1 instead of
/dev/dri/card2.

This branch fixes that, by adding support for querying udev
to determine the available drm devices.

As part of this effort, some clean up was performed:

1) a bunch of bit rotted tests were removed
2) large chunks of code were moved from main.c to helper
objects implemented in other files.
3) Other parts of main.c were moved around or refactored
so they were easier to read.

Based on work from Kevin Murphy <kemurphy.cmu@gmail.com>

https://bugs.freedesktop.org/show_bug.cgi?id=25943