+ DHCP: add ra_slaac_only, ra_allow_slaac config settings

Close # 2076

Squashed commit of the following:

commit 7a938fc6ffa0adc1e705f59778d7e7442b83d4b9
Merge: 3db2605e 6222d17d
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Thu Sep 24 16:34:23 2020 +0300

    Merge remote-tracking branch 'origin/master' into 2076-dhcp-ra-slaac

commit 3db2605efda79e8d361fb46c32bb9ec753de0e02
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Thu Sep 24 16:21:36 2020 +0300

    minor

commit de31c6aeebd78dce54ca54fbff010f3dd50ce974
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Thu Sep 24 12:23:09 2020 +0300

    don't initialize DHCPv6 server if we must force the clients to use SLAAC

commit 7c4239899c85880641dc5b4826ae34386bc5ee94
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Tue Sep 22 19:34:27 2020 +0300

    minor

commit 2b57688ce4c31d45d2236bd397e0b3041dac68c1
Author: Simon Zolin <s.zolin@adguard.com>
Date:   Mon Sep 21 19:50:11 2020 +0300

    + DHCP: add ra_slaac_only, ra_allow_slaac config settings

dhcpd/README.md

@@ -30,6 +30,10 @@ To set up a test environment for DHCP server you need:
 
 3. Start your VM, install an OS.  Configure your network interface to use DHCP and the OS should ask for a IP address from our DHCP server.
 
+4. To see the current IP address on client OS you can use `ip a` command on Linux or `ipconfig` on Windows.
+
+5. To force the client OS to request an IP from DHCP server again, you can use `dhclient` on Linux or `ipconfig /release` on Windows.
+
 ### Configure server
 
 1. Edit server configuration file 'AdGuardHome.yaml', for example:
@@ -37,12 +41,19 @@ To set up a test environment for DHCP server you need:
         dhcp:
           enabled: true
           interface_name: vboxnet0
-          gateway_ip: 192.168.56.1
-          subnet_mask: 255.255.255.0
-          range_start: 192.168.56.2
-          range_end: 192.168.56.2
-          lease_duration: 86400
-          icmp_timeout_msec: 1000
+          dhcpv4:
+            gateway_ip: 192.168.56.1
+            subnet_mask: 255.255.255.0
+            range_start: 192.168.56.2
+            range_end: 192.168.56.2
+            lease_duration: 86400
+            icmp_timeout_msec: 1000
+            options: []
+          dhcpv6:
+            range_start: 2001::1
+            lease_duration: 86400
+            ra_slaac_only: false
+            ra_allow_slaac: false
 
 2. Start the server