small upgrade progress reporting fixes

* Add a _Static_assert (C11 keyword) to make sure our percentages add up
* Correctly update prevpercent in setup_transaction() so we only send
  plymouth progress when it's relevant
* Initialize prevpercent in rpm_trans_callback() for safety
* Update some comments based on some timing observations
fix install counting

The existing code was doing installed++ whenever we hit inst_close_file,
which works fine.. unless a package has a %pretrans script, then we'll
open/close it before we do inst_start.

Which means we end up counting like this:

  <3 %pretrans scripts run>
  <install starts>
  [4/1588] (6%) installing libgcc-4.9.1-11.fc21...
  [5/1588] (6%) installing fontpackages-filesystem-1.44-10.fc21...
  <etc.>
  [1590/1588] (76%) installing ipw2100-firmware-1.3-17.fc21...
  [1591/1588] (76%) installing iwl5000-firmware-8.83.5.1_1-39.fc21.1...

We're off by three because of the three inst_close_file calls before
the first inst_start.

So, instead: count in inst_start normally, or inst_close_file during a
test transaction (which never does start/stop, but does exactly one
open/close per package).
Bump version, update spec changelog

Prepare for 0.9.0 release! fedup-0.9.0 will appear soon. IT IS COMING.