refactor: Rename subprocess.hpp to follow our header name conventions

[hebasto]

This PR renames the header *.hpp --> *.h and adjusts the header guard name, which makes it available for processing by linters.

Fixed the following linter warning:

The locale dependent function strerror(...) appears to be used:
src/util/subprocess.h:    std::runtime_error( err_msg + ": " + std::strerror(err_code) )

Unnecessary locale dependence can cause bugs that are very tricky to isolate and fix. Please avoid using locale-dependent functions if possible.

Advice not applicable in this specific case? Add an exception by updating the ignore list in /bitcoin/test/lint/lint-locale-dependence.py
^---- failure generated from lint-locale-dependence.py

[DrahtBot]

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ACK	TheCharlatan
Concept ACK	theStack

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Conflicts

No conflicts as of last run.

[fanquake]

Why not put the windows change with the windows change: #29868 ?
Looks like the other change could just be PR'd on it's own, and doesn't need to be based on anything?
Avoiding conflicts doesn't seem necessary given this branch already conflicts as-is?

[hebasto]

Why not put the windows change with the windows change: #29868 ? Looks like the other change could just be PR'd on it's own, and doesn't need to be based on anything? Avoiding conflicts doesn't seem necessary given this branch already conflicts as-is?

Thanks! Reworked per your suggestions.

[hebasto]

Drafted again until resolving a linter warning. (The header is processed by linters now).

[DrahtBot]

🚧 At least one of the CI tasks failed. Make sure to run all tests locally, according to the
documentation.

Possibly this is due to a silent merge conflict (the changes in this pull request being
incompatible with the current code in the target branch). If so, make sure to rebase on the latest
commit of the target branch.

Leave a comment here, if you need help tracking down a confusing failure.

_{Debug: https://github.com/bitcoin/bitcoin/runs/24018238387}

[hebasto]

Drafted again until resolving a linter warning. (The header is processed by linters now).

Linter warnings have been resolved. But the PR remains as a draft because the #29865 will make the diff smaller.

[hebasto]

Rebased and undrafted.

[theStack]

Concept ACK

[TheCharlatan]

Resolved linter warnings.

Can you mention this in the PR description?

[hebasto]

Resolved linter warnings.

Can you mention this in the PR description?

Thanks! The PR description has been updated.

[TheCharlatan]

ACK 08f756b