add optional support to replace argv[0] (fixes #472)

[Mattie (mattieb)]

Fixes #472.

With this, specifying the new property "argv0" in options will replace argv[0] with a custom string.

This is useful, for example, when starting a login shell:

const pty = spawn('/bin/zsh', [], {
  argv0: '-zsh'
});

If this property is omitted, the file parameter is used instead, which is identical to existing behavior.

[Mattie (mattieb)]

@microsoft-github-policy-service agree

[Mattie (mattieb)]

Hey y’all, let me know if anything’s missing, but I’ve been using this for quite some time now day-to-day and haven’t seen any issues.

[Mattie (mattieb)]

Hey folks, sorry for the churn today.

I've been keeping this PR up-to-date with main for 1½ years now, while the code around what I've changed has been evolving.

I've most recently done this because I was running package updates in my project that uses this support and needed to pull in the upgrades y'all have been making.

Is there a chance we could evaluate this relatively simple PR and merge it? I think it's a feature that many folks would find useful.

[Daniel Imms (Tyriar)]

Robo (@deepak1556) could you review when you get a chance?