Well, sometimes you just have to go for the hack. Trying to find a way
to add additional options to git-annex-shell without breaking backwards
compatibility, I noticed that it ignores all options after `--`, because
those tend to be random rsync options due to the way rsync runs it.

So, I've added a new class of options, that come in between, like
`-- opt=val opt=val ... --`

The parser for these will not choke on unknown options, unlike normal
getopt. So this let me add the additional info I needed to
pass to git-annex-shell to make it record transfer information. And
if I need to pass more info in the future, that's covered too.

It's ugly, but since only git-annex runs git-annex-shell, this is an
ugliness only I (and now you, dear reader) have to put up with.

Note to self: Command-line programs are sometimes an API, particularly
if designed to be called remotely, and so it makes sense consider
whether they are, and design expandability into them from day 1.

---

Anyway, we now have full transfer tracking in git-annex! Both sides of
a transfer know what's being transferred, and from where, and have
the info necessary to interrupt the transfer.

---

Also did some basic groundwork, adding a queue of transfers to perform,
and adding to the daemon's status information a map of currently running
transfers. 

Next up: The daemon will use inotify to notice new and deleted transfer
info files, and update its status info.
