Fix isAllowedDir matching by prefix

[sean-breen]

Proposed changes

Stops isAllowedDir function from matching by prefix. Originally this function would allow any path which was prefixed by an allowed directory, i.e

• if /var/log/nginx was present in allowed dirs, then the path /var/log/nginx-test would also be allowed. Not good!

This PR stops this match from taking place, but still matches if the provided path is a subdirectory of an already allowed directory, i.e

• if /var/log/nginx is in allowed dirs, /var/log/nginx/mysubdir will be allowed but /var/log/nginx-test will be blocked.

Checklist

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• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13