Add a CAF list for GHCI.

Retaining all looked-up symbols in a list in the interpreter was the
Wrong Thing To Do, since we can't guarantee that the transitive
closure of this list points to all the CAFs so far evaluated (the
transitive closure gets smaller as reachable CAFs are evaluated).

A Better Thing To Do is just to retain all the CAFs.  A refinement is
to only retain all CAFs in dynamically linked code, which is what this
patch implements.