bectl: Add -n flag for the activate subcommand to prevent it from promoting cloned datasets

[arrowd]

Summary

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

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While at it I noticed that running make -C sbin/bectl makes it load /usr/include/be.h instead of lib/libbe/be.h. Not sure if this should be fixed.

[markmi_dsl-only.net]

It might be appropriate for sbin/bectl/bectl.8 to now explicitly mention the default implicit promotion in its activate description. As I remember, the historical text was not explicit at all about what to expect related to what promotion controls. But, with the change, both alternatives become possibilities.

[arrowd]

• Reformulate man page.

[arrowd]

I get it all wrong from the beginning. There is no need in this flag.