(PUP-2889) Remove dependency on win32-eventlog constants

Previously, we were referencing win32-eventlog constants. However, version
0.6.0 introduced a backwards incompatible change by renaming the event
type constants[1]. As result, we can't simply upgrade to the latest ffi
gem.

This commit defines the constants in puppet, and removes our dependence on
the gem. It also adds eventlog specs, in particular to ensure that
Puppet.features.eventlog? is always true on windows.

[1] https://github.com/djberg96/win32-eventlog/commit/99aeb4d854306a6db699550884f189cbd3c8928a