Add *FUZZY-DUPLICATE-SYMBOL-FILTER*.

* slynk-fuzzy.lisp: Add *FUZZY-DUPLICATE-SYMBOL-FILTER* variable
that controls the removal of symbol name duplicates occurring
during fuzzy matching.

See https://github.com/slime/slime/pull/215. Cherry picked from Ivan
Shvedunov's commit 7f53acc76fa68bd77ce523d1f26179e1659125d6 in his
SLIME fork.

Conflicts:
	contrib/ChangeLog