Add option for forcing

[jas01]

Add a option to the mkdhcp command to allow to forcing the generation of the dhcpd-gen.conf event if they are no modification in the database on the server.

This option is usefull for us in two cases

1. During the prototyping of dhcp/netmagis, need some test without any modification to add to the DNS database.
2. More important, we have two differents geographic sites, and only one instance of netmagis, so we need at least two differents dhcpd servers (one on each site). In that case without this option it's the first cron who launch the mkdhcp who "win" the second never get the update.

Regards