Allow to override user agent with CURL_OPTS

[raphaelm]

Apparently, there is at least one CA out there where the WAF blocks requests using "dehydrated" in the User-Agent. I could try to bypass this with setting a different user-agent in CURL_OPTS, if the evaluation order was different, since curl man page states:

If --user-agent is provided several times, the last set value is used.

Context:

raphael ~ $ curl "https://acme-api.actalis.com/acme/directory"
{"newAuthz": "https://acme-api.actalis.com/acme/new-authz", "newNonce": "https://acme-api.actalis.com/acme/newnonce", "newAccount": "https://acme-api.actalis.com/acme/newaccount", "newOrder": "https://acme-api.actalis.com/acme/neworders", "revokeCert": "https://acme-api.actalis.com/acme/revokecert", "keyChange": "https://acme-api.actalis.com/acme/key-change", "renewalInfo": "https://acme-api.actalis.com/acme/renewal-info", "meta": {"home": "https://www.actalis.com", "author": "Actalis S.p.A. <info@actalis.com>", "name": "acme-server", "version": "1.1.3", "termsOfService": "https://www.actalis.it/acme/terms", "externalAccountRequired": true}, "a4f79e25fe3041b5b73d7b6308f8b94b": "https://community.letsencrypt.org/t/adding-random-entries-to-the-directory/33417"}
raphael ~ $ curl -A "dehydrated/1" "https://acme-api.actalis.com/acme/directory"
…<h3>Web Page Blocked!</h3><div class="notice"><p>The page cannot be displayed. Please contact the administrator for additional information.</p><p>URL: acme-api.actalis.com/acme/directory<br /><br/>Client IP: xx.xx.xx.xx<br/>Attack ID: 20000051<br/>Message ID: 001083935386</p><p></p></div></div></body></html>