/cc @0intro

This restriction is not the only difference between Plan 9 and other
operating systems in the os.Rename function.

For example, on Plan 9 you can only rename a file located
in the same directory.

Should we change os.Rename to handle both of these cases like other operating systems?

Renaming a file to a different directory could be done by copying the
file and removing the source file. However, if the source file is a directory,
the situation would be a bit more complex.

CL https://golang.org/cl/18291 mentions this issue.