Ensure license headers match the Cargo manifest licenses

[rzadp]

• Closes Compare license to Cargo.toml license-scanner#44
• Silent because only comments are changed in the crates.

What's inside

First, we change the file traversal mechanism from shell globbing to walking through files which happens inside the license-scanner - so it has knowledge about directory structure and can correlate files with corresponding Cargo manifest.

Next, added MIT-0 and Unlicense to the allowed list of licenses.
Unlicense appears only once across {Substrate,Cumulus,Polkadot} - in penpal-runtime.

Finally, updated headers in files that do not match the corresponding manifest license.