BLFS: remove only "a" lines for runtime deps
Presently, when the <node>groupxx file is created, we copy "a"
lines from <node> to <node>groupxx (changing "a" to "b") and
we remove all the lines containing the runtime dependency from
<node>. This may not always be what we want: for example, a
dependency can be both "required at runtime" and "optional for
tests". We should keep this information when cleaning the nodes.
Only when removing circular paths should the optional tie be broken
if necessary.
