handle: Add CURL* and CURLcode vars to struct

handle: Add CURL* and CURLcode vars to struct

Adding the CURLcode is necessary in order to return an error string from
pm_error. Unlike libfetch, curl returns numerical error numbers and does
not maintain a staticly allocated string with the last error generated.

Adding the curl object itself to the handle is advantageous (and
encouraged by curl_easy_perform(3)) because the handle is reusable for
successive operations. This cuts back on overhead when downloading
multiple files in a single transaction.

Signed-off-by: Dave Reisner <d@falconindy.com>