Add quick docs for KRB5_RESPONDER_QUESTION_PKINIT

Add some examples for KRB5_RESPONDER_QUESTION_PKINIT to the header
files.
Make the text of NSS's prompts look like OpenSSL's

When built with NSS, make the text of prompts that we issue to the user
better mirror the text we use when we build with OpenSSL.
Learn to prompt for passwords for PEM files

When we don't supply our own callback to PEM_read_bio_PrivateKey(), the
library uses its own, which interacts with the user via the controlling
terminal.  Here we provide a callback that calls the prompter instead.
Rename a couple of parameters for readability

We'll be adding another variable named "identity" later, to track the
name of the identity, so go ahead and give "label" and "id" names which
make it clearer that they're not identity names, but attributes of the
identity which were supplied by the caller.
Include an error name in one trace error message

When we fail to sign data, include the name of the library error if
we're built with debugging support.
Only call SEC_PKCS12DecoderFinish() once

We already call SEC_PKCS12DecoderFinish() before entering the switch()
statement, so don't call it again.  Failures to parse PKCS12 data due to
reasons other than not having a decryption password probably didn't end
well before now.
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 or PIN for client identities.  Otherwise, it should go ahead
and use a prompter to prompt for that information.
Traverse tokens like we do with OpenSSL for NSS

When built with NSS, change our token traversal strategy to mimic the
way we do things when we're using OpenSSL: forget about slot names, and
only look at the specified token, or the first one if one isn't
specified.
Add support for tracking deferred identity prompts

Track a list of deferred identities, for which we want to prompt for
passwords or PINs, in pkinit_identity_crypto_context structures, along
with associated warnings.  These are opaque outside of
pkinit_crypto_openssl and pkinit_crypto_nss, so both need to provide
functions that can be called from elsewhere to populate and query the
lists.
Add support for 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, it's not going to be called if
its result is not needed.  Otherwise, check for a password that's been
stashed for its use for that identity, and return a copy of it if it's
found.  If none of that works, try to use the prompter callback.

In various places where we attempt to load identity information, ignore
failures so yield new information if we're deferring prompts.
Add support for 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.
Pass deferred identity prompts to the responder

Use the list of deferred identity prompts and warnings, which we have
after calling pkinit_identity_initialize(), to build a list of questions
for responder callbacks to answer, and before calling
pkinit_identity_prompt() to actually load identities that are protected,
save any passwords and PINs which a responder callback may have
supplied, in case there's no prompter callback.
Pass token warnings flags to the prompter callback

Interpret the token flags and pass a warning about the state of the
token's PIN to the prompter function as part of prompting for the PIN.
When DEBUG=1, log why we fail to log in to a token
Pass a distinguishable identity name to responders

When using a PKCS#11 token, build an identity name using the token name
and slot ID, and pass that to the password prompter, so that even if the
user or configuration didn't originally specify either, we can be more
sure that we're applying any values we get back to the right token.
Rework loading of PEM credentials

Change the way the NSS implementation loads PEM-format credentials:
* When loading a certificate and a key, switch to loading the
  certificate first, to work around a bug in the PEM module.
* Support encrypted keys.
* Be smarter about finding the certificate after we've loaded it.
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 (warning: this way, only one response goes back)

We might want to change krb5_responder_pkinit_set_answer() to provide a
way of providing multiple PINs before committing to it.
Add a script for generating test PKI credentials
Add a helper for checking responder challenges

This test helper takes multiple options and one argument.

./misc/test_responder [options] principal
 -X preauth_option -> preauth options, as for kinit
 -x challenge      -> expected responder challenge, of the form
                      "question=challenge"
 -r response       -> provide a reponder answer, in the form "question=answer"
 -c                -> print the pkinit challenge
 -p identity=pin   -> provide the pkinit answer, in the form "identity=pin"
 principal         -> client principal name

If the responder callback isn't called, that's treated as an error.
If the responder callback is called and the challenge associated with
the specified question doesn't parse as JSON, a literal string compare
is done, otherwise both values are parsed as JSON and then re-encoded
before comparison.  In either case, the comparison must succeed.  Any
missing data or mismatches are treated as errors.
Add various PKINIT-with-responder tests