To have service working nicely with procd it should be running in the
foreground. Otherwise it's not possible to e.g. stop it with the init.d
script. Luckily for us pptpd has a simple switch that allows it.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Fixes: 15e7f611af ("pptpd: convert init script to procd")