ac_fs: Re-work ac_fs_mkdir_p()

[ac000]

Avoid prepending a '/' to absolute pathnames.

Also use fstatat(2) to check if the current directory component exists and if so skip the mkdirat(2).

[Copilot]

Pull Request Overview

This PR refactors the ac_fs_mkdir_p() function to improve handling of absolute pathnames and optimize directory creation by checking for existing directories before attempting to create them.

Key changes:

• Fixes handling of absolute paths by avoiding prepending '/' when the path already starts with '/'
• Adds fstatat(2) check to skip mkdirat(2) calls for existing directories
• Optimizes the directory creation process by reducing unnecessary system calls

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[Copilot]

Pull Request Overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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