Set the payprocd socket permissions.

* src/payprocd.c (create_socket): Set socket permission.

We want that the created socket is 0770 so that only the group can use
it. The owner of the socket is determined from the directory which
should have the sticky bit set for the group. For example by using

mkdir -p /var/run/payproc
chmod g+ws /var/run/payproc
chown payprocd:www-data /var/run/payproc