Call the public kadm5 API functions instead of the internal ones

Call the public kadm5 API functions instead of the internal ones

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.