Refuse to run if the last bit of the destination path contains a # character.

Refuse to run if the last bit of the destination path contains a # character.
This is a silly workaround for a rather serious bug in MacOSX: if you take
a long filename and convert it to an FSSpec the fsspec gets a magic
cooky (containing a #, indeed). If you then massage the extension of this
fsspec and convert back to a pathname you may end up referring to the
same file. This could destroy your sourcefile. The problem only occcurs
in MacPython-OS9, not MacPython-OSX (I think).

Closes bug #505562.