Allow client to send POST request

I have a requirement to initiate a POST request from an application using civetweb.  mg_download() could almost be coerced into doing this but it was waiting for a response before sending the data which caused deadlock unless we expected 100-continue (but this did not work through all proxies we tried).

So, I have modified civertweb to expose the mg_connect(), mg_printf() and mg_getreq() functions.  This allows me to use these lower level functions to implement the POST request.

Please feel free to do with this PR as you see fit.