libsmartctl: Virtual Client Interface

• Virtualize public interface for flexibility and ease of testing.
• Pure virtual libsmartctl::ClientInterface for isolated testing.
• Consistent snake cased naming conventions.

Rebase with upstream Release 6.6

git commit hash: 3701911

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk/smartmontools@4594

libsmartctl - client initialization no longer throws

• Client initialization used to throw a std::runtime_error if SMART interface initialization or drive database initialization fails. That was a design flaw, as an initialization failure is not an exceptional event. Exposed interfaces now return a CLIENTINITIALIZTIONFAILURE when called if an initialization failure occurs.

libsmartctl: ATA device vendor attributes minor change

• Remove decimal padding for vendor attribute fields value, worst, and threshold of ATA devices.

libsmartctl - error string API and minor refactor

• libsmartctl::errStr function to retrieve std::string representation of ctlerr_t.

• Minor refactor to hide unnecessary implementation details in public header file.