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.