dnsmasq: support dhcp_option config as a list

Configuring dhcp_option as an option does not allow the usage of white
spaces in the option value; fix this by supporting dhcp_option as a list
config while still supporting the option config to maintain backwards
compatibility

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>