Revision history for Keyring::Auto
+ Terminology fixes in docs and param names (realm instead of service/domain)
+ Recommended modules are picked according to the current platform.
+ Support for loading OSXKeychain backend on Mac OS/X (darwin) platform.
+ Added standalone non-trivial example.
+ get_keyring is exported by default.
+ Improved keyring selection code structure (introducing concept
+ Handling PASSWD_KEYRING_AUTO_DEBUG variable, if it is set, we
+ emit some diagnostics of what we pick.