Additional common environment variables:

* NO_COLOR -- disable messages colouring (see [cmd/Progress])
* REDO_TOP_DIR -- do not search for .do above that directory
                  (it can contain .redo/top as an alternative)
* REDO_NO_SYNC -- see [cmd/Fsync]
* REDO_INODE_TRUST -- see [OOD]
* REDO_MAKE -- see [Jobserver]
* REDO_SHELL -- path to the shell invoked instead of /bin/sh