Preferred content control is wired up to `--auto` and working for `get`,
`copy`, and `drop`. Note that `drop --from remote --auto` drops files that
the remote's preferred content settings indicate it doesn't want;
likewise `copy --to remote --auto` sends content that the remote does want.

Also implemented `smallerthan`, `largerthan`, and `ingroup` limits,
which should be everything needed for the scenarios described in
[[transfer_control]].

Dying to hook this up to the assistant, but a cloudy day is forcing me to
curtail further computer use.

----

Also, last night I developed a patch for the hS3 library, that should let
git-annex upload large files to S3 without buffering their whole content in
memory. I have a `s3-memory-leak` in git-annex that uses the new API I
developed. Hopefully hS3's maintainer will release a new version with that
soon.
