Deprecate the system directory accessors that take arguments

They are fundamentally broken due to their dependence on cwd and $PATH,
which may be changed after application startup.  Now that Application
stores an absolute argv0, the argument-less versions can provide a more
consistent answer.

Implementations for the functions are still the same, as they behave
correctly when an absolute argv0 is passed in.  At some point in the
future I'll remove the old versions completely.