isc-dhcp: integrate IPv4 DHCP service with procd and netifd

isc-dhcp: integrate IPv4 DHCP service with procd and netifd

Convert init-script to procd and allow to configure
isc-dhcp-server via UCI. Allow most by-network and by-host options
supported by dnsmasq.
User-defined dhcp-options are not supported yet, neither are tags.

Existing configurations with use-edited /etc/dhcpd.conf are still
respected, hence to enjoy the new features you have to migrate
your configuration to UCI and delete /etc/dhcpd.conf.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>