mktemp permission issue, fix

Hello,

We tried out the latest version of s3cmd and encountered an error with respect to file permissions due to the owner-restricted nature of mktemp.  I put together an update which uses os randoms to generate temp file names instead, which allows the permissions on the generated files to reflect the user's umask.  I also added to the existing error handling in order to clean up temp files.

We're looking forward to using s3cmd 1.5 and hope that this permission issue can be resolved either with this patch or with another approach.

Thanks,

Jason