Add --saturnin-get-subcommands devel key

A less intrusive replacement for _ls_sc subcommand

src/saturnin.sh.skel

         cache-home)     echo "$SATURNIN_CACHE_HOME"         ;;
         libexec)        echo "$SATURNIN_LIBEXEC"            ;;
         libexec-prefix) echo "$SATURNIN_LIBEXEC_PREFIX"     ;;
+        subcommands)    saturnin__lssc                      ;;
         *)              warn "unknown devel key: $key"
                         exit "$EXIT_USAGE" ;;
     esac