In fire_upload_buffer(), upload files in a loop.

This edit fixes the problem I had in #102 . With a list of 3955 files, curl thinks the argument list is too large, and I tend to agree. In fire_upload_buffer(), a loop over CURL_UPLOADS calls curl for every file. It might not be as efficient, but it works. It would be better to partition the list in chunks of e.g. 100 files, but that exceeds my skills in bash programming.

This edit also names the curl binary in a variable CURL_BINARY.