Change current buffer's name to NEWNAME (a string).
If second arg UNIQUE is nil or omitted, it is an error if a
buffer named NEWNAME already exists.
If UNIQUE is non-nil, come up with a new name using
Interactively, you can set UNIQUE with a prefix argument.
We return the name we actually gave the buffer.
This does not change the name of the visited file (if any).
(fn NEWNAME &optional UNIQUE)