package/dhcp: fix SysV init scripts output text

Fix various messages displayed by these scripts:
 - make start-stop-daemon quiet in order to avoid extra messages like
   "stopped /usr/sbin/dhcpd (pid 174)" being output between the command
   description and its result,
 - fix the script names in the usage strings.

Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>