(PUP-2521) Define Process constants locally

• Previously, Process constants were used in a few locations throughout the code. Capture the needed PRIORITY_CLASS constants in our local Windows Process class, and also define CREATE_NEW_CONSOLE. These constants have moved in new versions of the win32-process gem.
• Note that ext/windows/service/daemon.rb doesn't load Puppet code. The daemon only loads what it explicitly needs, so ensure the CREATE_NEW_CONSOLE constant is defined there.