Revert File::Copy::copy() to fail when copying a file onto itself

Revert File::Copy::copy() to fail when copying a file onto itself

Copying a file onto itself was made a fatal error by 96a91e0163.
This was changed in 754f2cd0b9 from an undesirable croak() to return 1,
but the documentation was never changed from it being a fatal error.
It should probably have remained an error as per the documentation (but
updated not to say fatal) for consistency with cases of copying a file
onto itself via symbolic links or hard links.