(one can still override checks using option '-f')

(option '-m' becomes option '-w' to zero labels entirely as was done before)

(an exported pool already needs option -f to clear labels: we cannot use a
single -f to handle invalid labels as well)

- this removes compilation warnings about static unused abd_* functions related
to inclusion of sys/vdev_impl.h.
- while here, also remove definition of -unused- VDEV_BEST_LABEL.

…l-labelclear-improvements