Add more comments to the library call patch

debian/patches/fix-library-calls.patch

-Description: Call the public API functions, not internal _c functions
+Description: Call the public kadm5 API functions instead of the internal ones
 Author: Russ Allbery <rra@debian.org>
 
+The kadm5_c_* functions are an internal API.  The public functions
+are the ones without the _c, which dispatch to the internal functions
+depending on whether one links with the client library or the server
+library.  By calling the _c functions, we don't get the benefit of
+prototype checking and fail if the internal function signature ever
+changes.
+
+The latter has now happened with Heimdal 1.6 currently used in Debian,
+and as a result the create_principal and chpass_principal functions
+here have stopped working because they provide the wrong number of
+arguments and the Heimdal library sees random stack garbage as the
+additional function arguments.
+
+Replace all C calls to use the versions without _c, since this module
+doesn't attempt to support the _s and _c interfaces simultaneously
+anyway and therefore no benefit accrues from calling the _c functions.
+This also fixes the backward-compatibility problem, since the public
+API did not change, only the internal function API.
+
 diff --git a/Kadm5.xs b/Kadm5.xs
 index f1312f8..8ee5d73 100644
 --- a/Kadm5.xs