stunnel: use the <pkg>_INSTALL_INIT_SYSV mechanism

Since the existing STUNNEL_INSTALL_CONF_SCRIPT hook no longer installs
the init script, we rename it to STUNNEL_INSTALL_CONF.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>