Resolve #647
Resolve #647

Implement a backoff approach to opening the system RNG: if opening read-write fails, try to open read-only. This will allow the RNG to be used, but attempts to add entropy will fail. If opening as read-only also fails, only then throw an exception.
webmaster128/botan,webmaster128/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,randombit/botan