Compatibility with JOpt 4.6

JOpt 4.6 redeclared its nonOptionArguments() method from List<String> to List<?>, requiring us to select String arguments only as we do for regular option values already.

Issue: SPR-11359
(cherry picked from commit 67e76e9)
Consistently declare @SuppressWarnings("serial") instead of dummy serialVersionUID
(cherry picked from commit 56b7d7a)
Downgraded Commons Pool dependency to 1.5.7

Commons Pool 1.6 introduced generics in its APIs, leading to either a 1.6 dependency at runtime or a load of raw type warnings in our CommonsPoolTargetSource.