Test suite: simplify and enhance getpid.sh

Test suite: simplify and enhance getpid.sh

- Try to only search for processes of the current user.
- Prefer using pgrep (in addition to pidof) when available.
- Streamline system overrides.
- Get rid of HEAD_FLAGS: all systems so far support "-1".
- Use "ps -o pid,comm" as the default, which is POSIX.1.
- Use "sort -r" to hopefully get the newest (=highest) PID, which is the
  case on older systems not using randomized PIDs at least.