SSL: implement public key pinning

Option --pinnedpubkey takes a path to a public key in DER format and
only connect if it matches (currently only implemented with OpenSSL).

Provides CURLOPT_PINNEDPUBLICKEY for curl_easy_setopt().

Extract a public RSA key from a website like so:
openssl s_client -connect google.com:443 2>&1 < /dev/null | \
sed -n '/-----BEGIN/,/-----END/p' | openssl x509 -noout -pubkey \
| openssl rsa -pubin -outform DER > google.com.der

lib/strerror.c

@@ -298,6 +298,9 @@ curl_easy_strerror(CURLcode error)
   case CURLE_NO_CONNECTION_AVAILABLE:
     return "The max connection limit is reached";
 
+  case CURLE_SSL_PINNEDPUBKEYNOTMATCH:
+    return "SSL public key does not matched pinned public key";
+
     /* error codes not used by current libcurl */
   case CURLE_OBSOLETE20:
   case CURLE_OBSOLETE24: