Add --cacert option like curl's frontend

[nwf]

SSL clients are always a real headache, sigh. Anyway, I find myself wanting to use httpdirfs with a self-signed cert that, for elaborate reasons, I cannot add to the system trust bundle. It looks like libcurl cannot fetch the bundle's location from the environment, so it would be nice to have a --cacert option that set the CURLOPT_CAINFO option to libcurl, like the curl frontend tool has.

One could imagine adding one for CURLOPT_PROXY_CAINFO, too.