dhcp: misc improvements to S80dhcp-server

This commit does misc improvements to the S80dhcp-server init script:

 - Use more variables: NAME, DAEMON, CFG_FILE

 - Read the configuration file in /etc/default/ in a more usual way
   (as done in S21rngd for example)

 - Remove leftover dhcpd3 string in the stopping action.

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