processhelp.pm: log taskkill pid info, add debug envs, enable in CI

processhelp.pm: log taskkill pid info, add debug envs, enable in CI

To debug the Windows CI fails further. Acting on the suspicions that
`taskkill` may sometimes be applied to the wrong process.

- log task info, and task child info before calling `taskkill` on a PID.
  (on native Windows.)
  One of the calls needs PowerShell.

- add env `CURL_TEST_NO_TASKKILL` to disable using `taskkill`.

- add env `CURL_TEST_NO_TASKKILL_TREE` to use `taskkill` without
  `-t`, meaning to kill the process, but not child processes.

- GHA/windows: disable `taskkill` calls, to see what happens.
  I'll revert or tweak this in a future commit depending on results.

Ref: https://github.com/curl/curl/discussions/14854#discussioncomment-13062859
Ref: https://github.com/curl/curl/discussions/14854#discussioncomment-14913014

Closes #19421