References to krb5_int16.rst and krb5_ui_2.rst were missing, but
recently began causing sphinx-build errors because Doxygen started
generating them after the stdint.h changes.

ticket: 7857

Some literal blocks in the new AEAD and IOV documentation in
gssapi.rst started with ":" instead of "::", causing documentation
build errors.

Add new types and constants to the apiref index files.

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

The addition of the KRB5_PADATA_AS_CHECKSUM macro in d7d74867952f caused
the doxygen bridge to emit a new RST file.  This file was not included in
the API reference toctree, causing a build failure in maintainer-mode.

Add an API to duplicate keytab handles, mirroring krb5_cc_dup.  Use it
to simplify the krb5 GSS acquire_cred code.

ticket: 7599 (new)

Rename krb5_free_ktypes to krb5_free_enctypes and add it to the public
API.

ticket: 7584