Don't use tmpnam

[jonathondgebhardt]

There is an edge case while using std::tmpnam and the compiler suggests using something else to mitigate this.

[jonathondgebhardt]

I took advantage of CURLOPT_WRITEFUNCTION and CURLOPT_WRITEDATA to write POST data to a buffer instead of writing straight to a temporary file. I tried this before, but did not handle the latter, so it kept crashing.