os: Rename does not overwrite existing files on Plan 9

[rsc]

TestRenameOverwriteDest should not need an exception for Plan 9.

[bradfitz]

/cc @0intro

[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.

[rsc]

Everyone expects that there are limitations on when you can rename across directories. Even Unix doesn't let you do that if directories are on different file systems. But people do expect rename in a single directory to work. I clarified in the docs.

[gopherbot]

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