Use macro for IANA assigned PA-AS-CHECKSUM number

Replace numeric value '132' by the macro KRB5_PADATA_AS_CHECKSUM
in preauth plugin.
Add a few comments to `PADATA types` in krb5.hin

Mostly, based on
http://www.iana.org/assignments/kerberos-parameters/kerberos-parameters.xml
Fix doc build

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.
Include default DIR::file ccache in collection

If the context's default ccache name is a subsidiary file of a
directory collection, include that single cache in the cursor walk
over the DIR type.

ticket: 7172
Add a krb5_db_check_pkinit_binding() to libkdb

Provide a way for kdb modules to decide whether or not a given client
certificate corresponds to a particular client principal.
Use krb5_db_check_pkinit_binding() in the KDC

If we can't find a matching SAN in the client's certificate, call
krb5_db_check_pkinit_binding() to check if the supplied certificate
matches the client in question.  If the kdb method doesn't return TRUE,
then we've failed.  This doesn't alter any other requirements we have
for client certificates.
Provide a check_pkinit_binding() method

Provide a check_pkinit_binding() function which checks the client's
directory server entry for a userCertificate value that's byte-for-byte
the same as the one we've been given to compare to.