Split pkinit_identity_initialize into two phases

Split part of pkinit_identity_initialize() into a second piece named
pkinit_identity_prompt().  Have each piece pass a new boolean flag to
crypto_load_certs() to indicate if it should defer prompting for a
password/PIN for client identities that require one.  If the flag isn't
set, then crypto_load_certs() should attempt to use a responder-supplied
value, or call the prompter if there isn't one.

ticket: 7680
Add support for PKINIT deferring identity prompts

Learn to manage a list of deferred identities, for which we want to
prompt for passwords or PINs, in pkinit_identity_crypto_context
structures, along with their associated token flags.  These are opaque
outside of pkinit_crypto_openssl and pkinit_crypto_nss, so both
implementations need to provide wrapper functions that can be called
from elsewhere in the module to populate and query the lists.

ticket: 7680
Use PKCS11_MODNAME for NSS PKINIT by default

Do what the OpenSSL-using code paths do, and load PKCS11_MODNAME if no
module is specified when we're told to use a PKCS11 identity.
Make the PKINIT NSS path also check for NULL certs

When called to free identity information, do what the OpenSSL-based
version does, and error out if the identity information is NULL.
Pass PKINIT identity prompts to the responder cb

Use the list of deferred identity prompts and warnings, which we have
after calling pkinit_identity_initialize(), to build a list of questions
to supply to responder callbacks.

Before calling pkinit_identity_prompt() to actually load identities that
are protected, save any passwords and PINs which a responder callback
may have supplied.

Because pkinit_client_prep_questions() can be called multiple times, and
we don't want to try to load all of our identities each of those times,
take some steps to ensure that we only call pkinit_identity_initialize()
and pkinit_identity_prompt() once per request.

ticket: 7680
Support PKINIT NSS deferred identity prompting

The password callback which we usually supply to NSS already gets a
pointer to the pkinit_identity_crypto_context structure, but it needs to
be passed the name of the identity for which it's being called.

If it gets a name, and it's deferring prompting, just add the identity
to the list of deferred identity prompts (the password callback wouldn't
have been called if its result wasn't needed), and either return NULL
(as an indication that we couldn't get a password) or an empty string (a
value which we know is invalid) if that's handier.

Otherwise, check for a password that's been stashed for its use for that
identity, and return a copy of it if one's found.  If none of that
works, try to use the prompter callback to ask for the password.

ticket: 7680
Support PKINIT OpenSSL deferred identity prompting

Add a password to the set of things that we can pass to a PEM password
callback and the function we use for loading PKCS12 bundles.  If we're
meant to defer identity prompts, just store the name of the identity
which we're loading.  Otherwise, if we're passed a password, use it.
Otherwise, use the prompter callback.

Add a password to the set of things that we can pass to the function
that we use for logging in to PKCS11 tokens, too, but if we're deferring
identity prompts, just return the identity name without doing anything
else.  If not, and we're passed a password, use that.  Otherwise, try to
use the prompter callback to get one.

ticket: 7680
Add non-JSON APIs for PKINIT responder items

Add wrappers for the JSON-oriented APIs for PKINIT responder items,
modeled after the API we provide for OTP items:

* krb5_responder_pkinit_get_challenge() returns the list of
  identities for which we need PINs
* krb5_responder_pkinit_challenge_free() frees the structure that
  was returned by krb5_responder_pkinit_get_challenge()
* krb5_responder_pkinit_set_answer() sets the answer to the PIN for
  one of the identities

[ghudson@mit.edu: style cleanup; added comment pointing to main body
of PKINIT module]

ticket: 7680
Add a helper for testing PKINIT and responder cb

ticket: 7680
Add tests for PKINIT using responder functionality

We now also test that the PKINIT challenge looks like we expect it to
look, that PKINIT fails if we don't provide a response or a prompter
callback, and that PKINIT succeeds with a response provided using either
the raw responder API or the PKINIT responder functions.

One thing that we don't check is which specific error code we get when
PKINIT fails:  the OpenSSL and NSS versions return different error codes
(some mixture of EIO, ENOMEM, ENOENT, and KRB5KDC_ERR_PREAUTH_FAILED)
when they encounter trouble loading client credentials.

ticket: 7680
Add the error to some PKINIT NSS debug messages
Teach the responder test helper about OTP
Test that password preauth works without PKINIT

Before we test authenticated PKINIT, slip in a test to check that
password-based preauthentication still works when the KDC is offering
PKINIT, but the client has no PKINIT credentials.