Minor fixes

[apocelipes]

• fix ffPathExists: only using stat makes FF_PATHTYPE_LINK never work under *nix systems. Some symlinks are linked to directories, so keep use stat when FF_PATHTYPE_LINK is not contained in type paramater for backward compatibility.
• ffWriteFileData only retries when error is ENOENT. Other errors such as EACCESS, ELOOP or EDQUOT also fail mkdir(), retrying after these errors is useless.
• fix readlink usage: readlink won't append a NUL, we should check the return value and add a NUL handly.

[apocelipes]

Would you tell why close this?

I think readlink's call is actually a bug.

[CarterLi]

See 4ae0c43