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

docs/curl.1

@@ -530,6 +530,19 @@ OpenSSL-powered curl to make SSL-connections much more efficiently than using
 
 If this option is set, the default capath value will be ignored, and if it is
 used several times, the last one will be used.
+.IP "--pinnedpubkey <pinned public key>"
+(SSL) Tells curl to use the specified public key file to verify the peer. The
+file must contain a single public key in DER format.
+
+When negotiating a TLS or SSL connection, the server sends a certificate
+indicating its identity. A public key is extracted from this certificate
+and if it does not exactly match the public key provided to this option,
+curl will abort the connection before sending or receiving any data.
+
+This is currently only implemented in the OpenSSL backend, with more backends
+expected to follow shortly.
+
+If this option is used several times, the last one will be used.
 .IP "-f, --fail"
 (HTTP) Fail silently (no output at all) on server errors. This is mostly done
 to better enable scripts etc to better deal with failed attempts. In
@@ -2180,6 +2193,8 @@ unable to parse FTP file list
 FTP chunk callback reported error
 .IP 89
 No connection available, the session will be queued
+.IP 90
+SSL public key does not matched pinned public key
 .IP XX
 More error codes will appear here in future releases. The existing ones
 are meant to never change.