Move extra/runner files to stbt-batch.d, without modification

Modifications will follow in subsequent commits, to keep the diffs
easier to follow.
Change "runner" references to "stbt batch"
Rename configuration section from `[runner]` to `[batch]`
Rename stbt batch server => stbt batch instaweb
Install the runner as "stbt batch"
stbt-completion: Tab-completion for `stbt batch`

* `stbt batch <TAB>` completes subcommand names (run, report, instaweb).

* `stbt batch run <TAB>` completes filenames (test scripts). It doesn't
  complete flags like `-1` or `-k`, nor arguments to the test script,
  nor separating test scripts with `--`.

* `stbt batch report <TAB>` completes directory names (it doesn't
  validate that the directory name is in the format generated by
  `stbt batch run`, because you don't know if it's just an intermediate
  directory on the path to the final test-run directory).

* `stbt batch instaweb <TAB>` offers "--help", "localhost:5000", and
  "0.0.0.0:5000". The latter two are just to give the user a hint; I
  don't know how to figure out a list of network interfaces, and
  offering all possible ports is pointless because you'll still have
  to type in all the numbers (there's no unambiguous port number to
  complete to).
test_stbt_batch_run_killtree: Fix when only running "test-stbt-batch.sh" test suite

You can run `tests/run-tests.sh` like this:

* `tests/run-tests.sh`: Will source all `tests/test-*.sh` files and run
  all the test cases therein.
* `tests/run-tests.sh test_stbt_batch_run_killtree`: Will source all
  `tests/test-*.sh` files and will run only the specified test case.
* `tests/run-tests.sh test-stbt-batch.sh`: Will source only the
  specified file, and will run all the test cases therein.

In that last case, `test_stbt_batch_run_killtree` was failing, because
`test_killtree` wasn't defined.
stbt, stbt-batch: exec, instead of fork, the subcommand

...such as `stbt-run` or `stbt-batch.d/report`.

The change to `stbt-batch` fixes the tests
`test_stbt_batch_run_sigint_{once,twice}`.

It doesn't make any difference when running `stbt batch run`
interactively and pressing Control-C. That always worked, presumably
because the shell sends the sigint signal to all foreground processes in
the process group.

The change to `stbt.in` (which becomes the installed `stbt` script)
isn't strictly necessary, but I don't think it will hurt. Less
intermediate processes in the PID tree can only be a good thing.
Merge branch 'install-the-runner'

Installs extra/runner scripts as `stbt batch`.

* install-the-runner:
  stbt, stbt-batch: exec, instead of fork, the subcommand
  test_stbt_batch_run_killtree: Fix when only running "test-stbt-batch.sh" test suite
  stbt-completion: Tab-completion for `stbt batch`
  Install the runner as "stbt batch"
  Rename stbt batch server => stbt batch instaweb
  Rename configuration section from `[runner]` to `[batch]`
  Change "runner" references to "stbt batch"
  Move extra/runner files to stbt-batch.d, without modification