Add a check for requirement 2

[tschmidtb51]

We should add a check for requirement 2 to the csaf_checker.

This might also be a question for the other programs...

[bernhardreiter]

The downloader shall reject non-compliant filenames.

[s-l-teichmann]

PR #330 adds this to the checker.

State of the other tools:

• The uploader checks the schema optionally. This is okay IMHO because:
• The validator checks the schema and the remote validator if configured.
• The provider checks the the schema and uses the remote validator if configured.
• The downloader checks the schema and the remote validator if configured,
• The aggregator checks the schema and the remote validator if configured.