os: missing ENOTEMPTY check in IsExist function

[perillo]

According to http://pubs.opengroup.org/onlinepubs/9699919799/functions/rename.html, the rename function may return EEXIST or ENOTEMPTY when "The link named by new is a directory that is not an empty directory".

On Linux 4.4.5 x86_64 I'm getting ENOTEMPTY, and so the os.IsExist function returns false. It should probably return true.

The same issue apply to the rmdir function.

A search on the online version of The Open Group Base Specifications Issue 7, reveals that ENOTEMPTY is only returned by the rmdir, rename, renameat, unlink and unlinkat functions.

[gopherbot]

CL https://golang.org/cl/21467 mentions this issue.