Fix a tiny memory leak in util_curl_init()

Each time a new Curl object is created util_curl_init() leaked
a small string buffer. The attached patch fixes the leak --
curl_easy_setopt() strdup's its argument; the caller does not need
to keep the buffer around for it.

https://sourceforge.net/p/pycurl/patches/13/
Merge pull request #35 from p-push/curl-init-memleak

Fix a tiny memory leak in util_curl_init()