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

tests/data/test2035

@@ -0,0 +1,43 @@
+<testcase>
+<info>
+<keywords>
+HTTPS
+HTTP GET
+PEM certificate
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+</reply>
+
+#
+# Client-side
+<client>
+<features>
+SSL
+</features>
+<server>
+https Server-localhost-sv.pem
+</server>
+ <name>
+HTTPS wrong pinnedpubkey but right CN
+ </name>
+ <command>
+--cacert %SRCDIR/certs/EdelCurlRoot-ca.crt --pinnedpubkey %SRCDIR/certs/Server-localhost-sv.der https://localhost:%HTTPSPORT/2035
+</command>
+# Ensure that we're running on localhost because we're checking the host name
+<precheck>
+perl -e "print 'Test requires default test server host' if ( '%HOSTIP' ne '127.0.0.1' );"
+</precheck>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<errorcode>
+90
+</errorcode>
+</verify>
+</testcase>