bug#11858: df -m undocumented, why no df -g

[Paul Eggert]

On 07/04/2012 01:11 AM, Andreas Jaeger wrote:
> df -k and df -m both work but only df -k is mentioned as part of df --
> help. So, the omission to document -m is IMO a bug.

I think the general idea is that -k was a mistake, but
it's standardized, and that we don't want to have
options -m, -g, -t, -p, -e, -z, -y for the other sizes
(among other things -t is already taken).  -m is there
only for BSD compatibility but we don't want to publicize
it and we may want to take it back at some point.