Backport of SF patch 590294: os._execvpe security fix (Zack Weinberg).

Backport of SF patch 590294: os._execvpe security fix (Zack Weinberg).

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.