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

src/tool_cfgable.h

@@ -110,6 +110,7 @@ struct OperationConfig {
   char *cacert;
   char *capath;
   char *crlfile;
+  char *pinnedpubkey;
   char *key;
   char *key_type;
   char *key_passwd;