Pull request: dhcpd: add purge, imp code

Updates #1691.

Squashed commit of the following:

commit 2ce6cc005d09ac7d63e4e575705d86e92046697f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jun 16 16:42:57 2021 +0300

    dhcp: imp code

commit 8f2bd7048d864e10aaed9e7338c0bbe852699a31
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jun 16 16:27:06 2021 +0300

    dhcpd: add purge, imp code

internal/aghnet/net.go

@@ -424,9 +424,3 @@ func CollectAllIfacesAddrs() (addrs []string, err error) {
 
 	return addrs, nil
 }
-
-// JoinHostPort is a convinient wrapper for net.JoinHostPort with port of type
-// int.
-func JoinHostPort(host string, port int) (hostport string) {
-	return net.JoinHostPort(host, strconv.Itoa(port))
-}