Replace direct registry access with odbccp32.dll offered functionality

The driver accesses the registry directly currently when a DSN name is received in the connection string.
This should be replaced with odbccp32.dll's [SQLGetPrivateProfileString()](https://docs.microsoft.com/en-us/sql/odbc/reference/syntax/sqlgetprivateprofilestring-function) & co. instead (for better portability also).