Don't need to clear the caller-specified one.

Necessary to preserve type information.  The try/catch sequence didn't
work previously.

Same fix as in commit 1c904000

For exception-safety.

Reduce some unnecessary overhead.

Replaces StatFile(), with a portable data object.

PathExists() should better do what CheckAccess() does, and
CheckAccess() doesn't do what its name implies.

The old struct config_param remains only for top-level string options.

Return false on error, not nullptr.

New in clang 3.6.