process group killing

This seems to work pretty well.

Handled the process groups like this:

- git-annex processes started by the assistant for transfers are run in their
  own process groups.
- otherwise, rely on the shell to allocate a process group for git-annex

There is potentially a problem if some other program runs git-annex
directly (not using sh -c) The program and git-annex would then be in
the same process group. If that git-annex starts a transfer and it's
canceled, the program would also get killed. May or may not be a desired
result.

Also, the new updateTransferInfo probably closes a race where it was
possible for the thread id to not be recorded in the transfer info, if
the transfer info file from the transfer process is read first.

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