Caddyfile parameter "clients" of "tls" henceforth accepts a special
first modifier. It is one of, and effects:

 * request         = tls.RequestClientCert
 * require         = tls.RequireAnyClientCert
 * verify_if_given = tls.VerifyClientCertIfGiven
 * (none)          = tls.RequireAndVerifyClientCert

The use-case for this is as follows: A middleware would serve items to the
public, but if a certificate were given the middleware would permit file
manipulation.

And, in a different plugin such as a forum or blog, not verifying a client
cert would be nice for registration: said blog would subsequently only
compare the SPKI of a client certificate.