Doc: Reorder roadmap
Provide all documented sub Environment variables
Stop using sub's name in environment vars

All subdue provided environment variables now share the `SUB_` prefix.
Originally the sub name was there so that nested sub invocations (yes,
that is possible, a command could be a sub on itself) would not confuse
the variables. However, by sharing the name, and making sure all of them
are set when running the driver, we can guarantee that the values of the
variables seen by the command have been set by the closes calling sub
driver in the call chain.
Doc: Fix env var name
Fix env var name generation

Was missing the trailing underscore.
Avoid using iteritems (no python 3 support)
Make _SUB_IS_EVAL_ work according to the docs
Test: Environment variables
Doc: Mark things as done
Doc: Start a FAQ