curl: allow "pkcs11:" prefix for client certificates

RFC7512 provides a standard method to reference certificates in PKCS#11
tokens, by means of a URI starting 'pkcs11:'.

We're working on fixing various applications so that whenever they would
have been able to use certificates from a file, users can simply insert
a PKCS#11 URI instead and expect it to work. This expectation is now a
part of the Fedora packaging guidelines, for example.

This doesn't work with cURL because of the way that the colon is used
to separate the certificate argument from the passphrase. So instead of

   curl -E 'pkcs11:manufacturer=piv_II;id=%01' …

I instead need to invoke cURL with the colon escaped, like this:

   curl -E 'pkcs11\:manufacturer=piv_II;id=%01' …

This is suboptimal because we want *consistency* — the URI should be
usable in place of a filename anywhere, without having strange
differences for different applications.

This patch therefore disables the processing in parse_cert_parameter()
when the string starts with 'pkcs11:'. It means you can't pass a
passphrase with an unescaped PKCS#11 URI, but there's no need to do so
because RFC7512 allows a PIN to be given as a 'pin-value' attribute in
the URI itself.

Also, if users are already using RFC7512 URIs with the colon escaped as
in the above example — even providing a passphrase for cURL to handling
instead of using a pin-value attribute, that will continue to work
because their string will start 'pkcs11\:' and won't match the check.

What *does* break with this patch is the extremely unlikely case that a
user has a file which is in the local directory and literally named
just "pkcs11", and they have a passphrase on it. If that ever happened,
the user would need to refer to it as './pkcs11:<passphrase>' instead.