Rationale for this change is following.

In my workflow I first create the recovery_point0 BE. But since it may be
booted in and changed somehow (and be broken), I also immediately create the
@clean snapshot.

Now if I want my default BE to be rolled back to the recovery point I run:

• bectl rename default removeMe
• bectl create -e recovery_point0@clean default
• bectl activate default

But the last step destroys the @clean snapshot making it
impossible to perform the same operation second time. I understand that it is
not really destroyed but moved to default and I can use it to recreate
recovery_point0@clean, but I feel that there should be a way to opt-out from
the promotion during the BE activation.

The flag name is subject for bikeshedding.

For the context, the change that introduced current behaviour is https://reviews.freebsd.org/D40972