Add validation to allowed directories on config load

[sean-breen]

Proposed changes

Adds some validation of paths when loading the allowed directories.

Allowed paths must:

• be absolute
• not contain any relative portions (.,..,etc)
• contain any unsupported characters

Any files not satisfying this criteria will be ignored, and logged at WARN level.

When checking if a path is allowed using isAllowedDir, we will now check if the path exists and if it does we will perform a symlink check. Any symlinks will be logged and the function will return false.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• 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