Fix confusing error message on macOS when running without sudo

[chiranjeevi-max]

Fixes #486

On macOS, when running bandwhich without elevated privileges, users see confusing error messages like:

lo0: No such file or directory (os error 2)
en0: No such file or directory (os error 2)

This happens because macOS returns ENOENT instead of EPERM when accessing BPF devices without permission.

with this PR, the program treats ErrorKind::NotFound as a permission error on macOS/FreeBSD, so users now see the helpful message:

Insufficient permissions to listen on network interface(s). Try running with sudo.