Introducing FLAVORS in the OpenJDK ports

Hi,

I've updated the java/openjdk25 port to use the FLAVORS mechanism to be
able to build variants of the port. This allows for building the
JRE variant, and headless variants of both the JRE and the full JDK.

However, this also introduces som issues that currently are being
discussed here:
https://reviews.freebsd.org/D53782

I've also written a bit about my thoughts on this in a post on my blog:
https://kodeknekkeriet.net/en/blog/introducing-flavors-in-openjdk-freebsd/

I'm not entirely sure this is the way to go, but I would really welcome
input from the community on this. Both users of the OpenJDK, and people
more familiar with the FreeBSD ports system than I am.

Thanks a lot!
Harald