process: move --help and --bash-completeion handling to startExecution

Because they are similar to `--prof-process` and are part of
the execution instead of initialization.
Also move the `getOptionValue` initialization to top scope since
it's used everywhere and add comments about the flags.

PR-URL: https://github.com/nodejs/node/pull/25262


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>