Client: Fix permissions on plugin helper directory.
The Makefile target "install-client-nokey" creates the plugin-helper directory /usr/lib/<ARCH>/mandos/plugin-helpers as mode u=rwx,go= (0700). Make this also the case for the Debian package. Also change the Makefile so it does not install the plugin helper "mandos-client-iprouteadddel" as setuid root; this is unnecessary and was, due to dh_fixperms, never propagated to the Debian package anyway.
* Makefile (install-client-nokey): Do not set setuid bit on "plugin-helpers/mandos-client-iprouteadddel". * debian/mandos-client.postinst (configure): If older version, fix permissions on plugin helper directory. * debian/rules (override_dh_fixperms-arch): Exclude plugin helper directory from dh_fixperms.