build-infra: Make JDK_BUILD_NUMBER and MILESTONE customizable

Currently, JDK_BUILD_NUMBER and MILESTONE are set in version.numbers. This is considered a shell script fragment that are executed, effectively overwriting any env variables set by the user. This makes MILESTONE and JDK_BUILD_NUMBER impossible to personalize.

I'm not convinced the best way of customizing this is by environmental variables. However, I don't think configure options are better, nor are make variables. :-(

Hm. Maybe the best way would be to have a configure flag pointing out an alternative version.numbers file...? (And we should REALLY reconsider that name. ".numbers" is *not* a good suffix for a file. Better use version-numbers in that case, the dot just makes the name awkward.)