1) Do not attempt to exec a file which does not exist
just to find out what error the operating system
returns.  This is an exploitable race on all platforms
that support symbolic links.

2) Immediately re-raise the exception if we get an
error other than errno.ENOENT or errno.ENOTDIR.  This
may need to be adapted for other platforms.

(As a security issue, this should be considered for 2.1
and 2.2 as well as 2.3.)