MFC r345130,r345184: trim(8): add another safety net and more user-friendly
error message in verbose mode.

MFC r343118: new small tool trim(8) to delete contents for blocks
on flash based storage devices that use wear-leveling algorithms.

Re-add new small tool trim(8) to delete contents for blocks
on devices using wear-leveling algorithms as a few weeks passed
after review and discussion of trim(8) ceased and
we still have no utility to perform the job.

Reviewed by: hackers@
MFC after: 2 weeks