Add asn1c test vectors for alg-agility types

For the test-vectors target in tests/asn.1, add ASN.1 modules from RFC
4556 and draft-ietf-krb-wg-pkinit-alg-agility-06.txt, and output test
encodings for PrincipalName, KRB5PrincipalName, OtherInfo, and
PkinitSuppPubInfo.  In the alg-agility module, AuthPack and DHRepInfo
are renamed, as asn1c otherwise rejects them as conflicting with the
RFC 4556 definitions.
Define SED in util/verto/Makefile.in

When the rules from util/k5ev/Makefile.in were moved to
util/verto/Makefile.in, the rule to make rename.h was broken on
non-gmake makes because the definition of SED wasn't carried over.
Define it now.
Removed unused leash help file

The file 'checkbox blank.png' is unneeded and contains a space in the
name, unnecessarily interfering with running find | xargs over the
source tree.  Remove it.

ticket: 7352 (new)
queue: kfw
target_version: 1.10.4
tags: pullup
Add zapfreestr() helper to k5-int.h

Since zapfree(str, strlen(str)) won't work for possibly-null values of
str, add a helper zapfreestr() which only calls strlen() if the string
value is non-null.
Add cred store functions to gssapi32.def

25ee704e83c2c63d4b5ecd12ea31c1979239041e added three new public GSSAPI
functions; add them to the DLL export file.

ticket: 7217
Add responder feature for initial cred exchanges

Add new APIs:
* krb5_get_init_creds_opt_set_responder
* krb5_responder_get_challenge
* krb5_responder_list_questions
* krb5_responder_set_answer

If a caller sets a responder, it will be invoked after preauth modules
have had a chance to review their incoming padata but before they produce
outgoing padata.  The responder will be presented a set of questions with
optional challenges.  The responder should then answer all questions it knows
how to handle.  Both the answers and the challenges are printable UTF-8 and
may contain encoded, structured data specific to the question asked.

Add two new callbacks and one optional method to the clpreauth
interface.  The new method (prep_questions) allows modules to ask questions
by setting them in the responder context using one of the new callbacks
(ask_responder_question).  The other new callback (get_responder_answer) is
used by the process method to read the answers to the questions asked.

ticket: 7355 (new)
Add internal base64 encoding and decoding support

Add base64 support based on Heimdal's libroken base64 code.
Add internal JSON encoding and decoding support

Add JSON support based loosely on Heimdal's heimbase code.
Introduce gss_export_cred and gss_import_cred

Add gss_export_cred and gss_import_cred mechglue functions to
serialize and unserialize GSSAPI credential handles.  Mechanism
implementations and tests will follow.

ticket: 7354 (new)
Add krb5 support for GSS cred export and import

Using the new internal JSON support to implement serialization and
unserialization of krb5 GSS credentials.

ticket: 7354
Add SPNEGO support for GSS cred export and import

ticket: 7354
Add tests for GSS cred export and import

ticket: 7354
Add new api to Sphinx doc; doc introduced version
Don't use kdc_active_realm in kdc/rtest.c

Too many parts of the KDC rely on the global variable
kdc_active_realm.  rtest.c doesn't actually need any of the
KDC-specific information that's avaiable through kdc_active_realm, so
delete references to it.
WIP (doesn't all build yet)
Eliminate some KDC globals

Make kdc_active_realm a local variable in every function that needs
it.  Pass it around in various state structures as needed.