rabbit_env: Record variables' origin

[dumbbell]

I.e. we record the fact that a particular value:

• is the default value, or
• comes from an environment variable, or
• comes from querying a remote node

This required a significant refactoring of the module, which explains the large diff.

At the same time, the testsuite was extended to cover more code and situations.

This work permits us to move remaining environment variables checked by rabbit to this module. They include:

• $RABBITMQ_LOG_FF_REGISTRY
• $RABBITMQ_FEATURE_FLAGS
• $NOTIFY_SOCKET

References rabbitmq/rabbitmq-server#2180.