BLFS Dependencies: Fix pass2 not run

With the way we manage "first" deps, it may happen that no node
unreference the pass2 node. In that case, we add it to root.
Add some documentation for what we do for those deps, too.

BLFS/libs/func_dependencies

@@ -298,6 +298,13 @@ clean_subgraph() { #
                 the name of the group is chosen so that it is unlikely as
                 a package name (so that it is removed when building the
                 xml book).
+                Also change the "first" qualifier so that a cycle:
+                 A -first-> B ---chain---> A becomes:
+                 B ---chain---> A -before-> B-pass1
+                and we remove all the dependencies which have a chain to
+                A in B-pass1.
+                Since we do not change anything else, it may happen that
+                nothing depends on B. In that case, B is appended to root.
     input       vars: None
                 files: <node>.dep files containing dangling edges and
                        "after" qualifiers
@@ -396,6 +403,9 @@ for node in $(grep -l ' f ' *); do
   } <$node
   for id_of_dep in $lines_to_change; do
     sed "/\ $id_of_dep\$/"'{s/[[:digit:]] f /1 b /;s/$/-pass1/}' -i $node
+    if ! grep -q " $id_of_dep\$" *.dep; then
+      echo 1 b $id_of_dep >>root.dep
+    fi
   done
 done
 } # End clean_subgraph