Bugfixes all day.

The most amusing bug, which I just stumbled over randomly on my own,
after someone on IRC yesterday was possibly encountering the same issue,
made `git annex webapp` go into an infinite memory-consuming loop on
startup if the repository it had been using was no longer a valid git
repository.

Then there was the place where HOME got unset, with also sometimes amusing
results.

Also fixed several build problems, including a threaded runtime hang
in the test suite. Hopefully the next release will build on all Debian
architectures again.

I'll be cutting that release tomorrow. I also updated the linux
prebuilt tarballs today.

----

Hmm, not entirely bugfixes after all. Had time (and power) to work
on the repository configuration form too, and added a check box to it that
can be unchecked to disable syncing with a repository. 
Also, made that form be displayed after the webapp creates a new
repository.
