Resync bash completion to include all flags defined in the code, and to autocomplete properly on all parameters that are reasonably possible to do so on today

This also includes several new minor features that are interesting, so do explore a little. :)

Finally, this also fixes a few bugs where commands would complete parameters that they won't necessarily accept.  We still have a few of these cases, but they're reduced to a minimum now.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Switch `docker run --username` to `docker run --user` which is more accurate (while we can still do so without breaking anyone's scripts)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)