Fix for bug reported by Bruno Haible cmucl-imp 2004-10-19:
... be more clear about when situations are name conflicts
... signal symbol-by-symbol rather than all at once, so that...
... a restart can be offered to resolve the conflicts in favour
of any of the conflicting symbols
... document the condition type, symbol list accessor and restart
... also frob the docstrings a little for sbcl house style