check_oracle plugin improvement

The Oracle plugin has no option for testing for a successful login.  The
closest option to this is --login, which uses a fixed set of credentials and
expects to receive the logon failure error code.

This patch adds an option called --connect.  This option takes a full SID and
returns success if it doesn't detect any error codes.  With this, we can check
that a database is allowing logins from valid credentials (and avoid filling up
DB logs with failed authentications).