Escape TLS certificate DNs that are invalid UTF-8

[timopollmeier]

What:
The subject and issuer DNs are checked whether they are valid UTF-8
before storing them in the database. If they are not, all non-ASCII
characters are escaped.

Why:
The invalid characters would cause SQL errors when then TLS
certificates are added at the end of a scan.

How did you test it:

• Created a self-signed certificate with the GnuTLS certtool
  where the subject contained an ISO 8859-1 encoded umlaut
• Ran a test HTTPS server with this certificate using gnutls-serv
• Scanned the host with the test server

Checklist:

• Tests
• CHANGELOG Entry