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