even if only defined to NULL. This means the accessors can always be
present.
Use RTLD_PRELOAD_VAR instead of _PATH_SUDO_NOEXEC to tell when
noexec is available.
Add ENABLE_SUDO_PLUGIN_API and use it instead of _PATH_SUDO_PLUGIN_DIR
to tell when the plugin API is available.
Add sudo_conf_clear_paths() to clear the path values so the
regress tests are not affected by compile-time settings.