implement dataUnits finally

implement dataUnits finally

Added support for "megabit" and related bandwidth units in
annex.stalldetection and everywhere else that git-annex parses data units.

Note that the short form is "Mbit" not "Mb" because that differs from "MB"
only in case, and git-annex parses units case-insensitively. It would be
horrible if two different versions of git-annex parsed the same value
differently, so I don't think "Mb" can be supported.

See comment for bonus sad story from my childhood.

Sponsored-by: Nicholas Golder-Manning