ipc: accept max-connections on bare unix -ipcbind address #36795

Description

This PR allows max-connections to work with a bare unix address in -ipcbind.

Previously, unix,max-connections=8 was rejected, while unix:,max-connections=8 worked.

This change makes both formats valid and consistent, making the option easier to use and avoiding unnecessary configuration errors.

Testing

• Added unit tests in ipc_tests.cpp. The tests pass with the fix and fail without it.
• Tested -ipcbind=unix,max-connections=8 on regtest and confirmed that it creates 8 connection slots.

Help text

The original help text now works as written, so the proposed help text change was removed.

Fixes #36657

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36795
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36795.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36795#author-ralph

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline and AI policy for information on the review process. A summary of reviews will appear here.

<!--174a7506f384e20aa4161008e828411d-->

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #31260 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/31260.svg">} (scripted-diff: Type-safe settings retrieval by ryanofsky)
• #17783 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/17783.svg">} (common: Disallow calling IsArgSet() on ALLOW_LIST options by ryanofsky)
• #17581 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/17581.svg">} (refactor: Remove settings merge reverse precedence code by ryanofsky)
• #17580 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/17580.svg">} (refactor: Add ALLOW_LIST flags and enforce usage in CheckArgFlags by ryanofsky)
• #17493 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/17493.svg">} (util: Forbid ambiguous multiple assignments in config file by ryanofsky)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

Thanks for looking into this. The doc change would help clarify things, but I’d prefer a change where the parser accepts unix,max-connections=x, with a test for it. Since unix already works for the default socket, I don’t see much benefit in requiring a colon just to limit the max connections. Supporting both forms would also make the existing help text work as written.

@enirox001 Thanks for the feedback, I've switched to fixing the parser and added a unit test to cover it. With this change, the original help text now works as written so there's no need to modify it.