dcron: provide init script

dcron doesn't create a pid file, so we run it in foreground but make
start-stop-daemon create the pidfile and move the daemon to background.

Give it a number greater than exim's init script, so job output email
can be sent.

[Thomas: as suggested by Yann E. Morin, use DCRON_INSTALL_INIT_SYSV.]

Signed-off-by: Alvaro G. M <alvaro.gamez@hazent.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>