scripts/shell{,.utils}: change some of session handling.
scripts/shell: allow parent stages to run.
scripts/shell: pass selective stage list to the sub-instances.
scripts/shell: improve selective stage list emptiness handling.
scripts/shell: fix session name handling.
scripts/shell: fix SUBSESSION_PREFIX.
scripts/shell: fix messages wrt. subsessions.
scripts/shell: allow running all child stages under specified name.