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>

net/isc-dhcp/files/dhcpd.defaults

@@ -0,0 +1,11 @@
+#!/bin/sh
+uci -q get dhcp.isc_dhcpcd && exit 0
+touch /etc/config/dhcp
+
+uci batch <<EOF
+set dhcp.isc_dhcpd=isc_dhcpd
+set dhcp.isc_dhcpd.authoritative='1'
+set dhcp.isc_dhcpd.default_lease_time='3600'
+set dhcp.isc_dhcpd.max_lease_time='86400'
+commit dhcp
+EOF