plugin interface functions must specify calling convention

plugin interface functions must specify calling convention

The plugin interfaces were originally implemented for use on
Unix where KRB5_CALLCONV, HEIM_CALLCONV and similar are defined
as nothing.  However, on 32-bit Windows the calling convention
matters and executing a __stdcall function through a __cdecl
function pointer will result in failures.

This change updates the krb5plugin_windc, krb5plugin_service_locate,
krb5plugin_send_to_kdc plugins to specify the KRB5_CALLCONV for
functions.   This brings the plugins into compliance with the
heim_plugin_common interface requirement that init() and fini()
use the platform specified HEIM_CALLCONV.

The krb5-plugin(7) man page is updated and the lib/krb5/test_plugin
test is also fixed.

With this change all tests pass on 32-bit Windows.

Change-Id: Ic9d2e1285c9c179e3898dc9d071ed092bcddc406