closes #443

- Show how a certificate can be obtained using OpenSSL.

Bug: https://github.com/bagder/curl/pull/430
Reported-by: Daniel Hwang

closes #376

Also add public key extraction example to CURLOPT_PINNEDPUBLICKEY doc.

Bug: https://bugzilla.redhat.com/1195771

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

Inludes all the sections to consider.