Rename current buffer to a similar name not already taken.
This function is useful for creating multiple shell process buffers
or multiple mail buffers, etc.
Note that some commands, in particular those based on `compilation-mode'
(`compile', `grep', etc.) will reuse the current buffer if it has the
appropriate mode even if it has been renamed. So as well as renaming
the buffer, you also need to switch buffers before running another
instance of such commands.