Set SSL_VERIFY_PEER to request a client certificate from the server,
when available. Have to shim the certificate verification function or
else it will fail on self-signed client certs.
In serve_cgi retrieve client certificate, create a fingerprint, and set
proper environment variables. It's pretty barebones, it doesn't parse
the certificate to give any other useful info like the common name, but
it's acceptable IMO. For most CGI uses the fingerprint is the only
thing that is needed anyways.
This takes the nginx approach to the "root" directive, which is simpler
to implement and more consistent with more complex routing behaviors
like regexp.
The path component of the URL is now simply appended to the root to form
the path to the file which should be served to the client.
