dnsmasq: add dhcp-script hook for other packages

Adds a script which acts as a hook for when dnsmasq creates/destroys a
lease, or completes a TFTP file transfer. The hook loops through scripts
in appropriate directories inside '/etc/hotplug.d', executing each one with
the same arguments supplied by dnsmasq.

In case dnsmasq is jailed by ujail the dhcp-script hook will not work as
expected as ujail does not yet support executing a script within a jail.

Signed-off-by: Nick Brassel <nick@tzarc.org>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>