open on nonexistent file returns wrong error

[mkroening]

open should return ENOENT if

O_CREAT is not set and a component of path does not name an existing file, or O_CREAT is set and a component of the path prefix of path does not name an existing file, or path points to an empty string.

It should return ENOENT or ENOTDIR if

O_CREAT is set, and the path argument contains at least one non- character and ends with one or more trailing characters. If path without the trailing characters would name an existing file, an ENOENT error shall not occur.

Currently, open("/a") returns EINVAL, if /a does not exist.