Pull request 1978: 4923 gopacket dhcp vol.2

Merge in DNS/adguard-home from 4923-gopacket-dhcp-vol.2 to master

Updates #4923.

Squashed commit of the following:

commit d0ef7d44af9790ed55401f6f65c7149f4c3658f7
Merge: f92b4c72d a4fdc3e8e
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Aug 30 13:43:41 2023 +0300

    Merge branch 'master' into 4923-gopacket-dhcp-vol.2

commit f92b4c72de03ceacb9b8890b7cf4307688795ce5
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Mon Aug 28 12:33:29 2023 +0300

    dhcpd: imp code

commit 63f0fce99a0343af2670943770cfef4694ae93ed
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Aug 24 15:01:34 2023 +0300

    all: imp dhcpd code

commit 563b43b4b5ab6c9c9046c7f09008ea3ef344f4e9
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jul 28 17:03:55 2023 +0300

    dhcpd: imp indexing

commit 340d3efa90ac4d34ba3d18702692de0fbc0247be
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Fri Jul 28 16:22:43 2023 +0300

    all: adapt current code

internal/dhcpsvc/dhcpsvc.go

@@ -53,7 +53,7 @@ type Interface interface {
 	// IPByHost returns the IP address of the DHCP client with the given
 	// hostname.  The hostname will be an empty string if there is no such
 	// client, due to an assumption that a DHCP client must always have a
-	// hostname, either set by the client or assigned automatically.
+	// hostname, either set or generated.
 	IPByHost(host string) (ip netip.Addr)
 
 	// Leases returns all the DHCP leases.
@@ -104,6 +104,9 @@ func (Empty) MACByIP(_ netip.Addr) (mac net.HardwareAddr) { return nil }
 // IPByHost implements the [Interface] interface for Empty.
 func (Empty) IPByHost(_ string) (ip netip.Addr) { return netip.Addr{} }
 
+// type check
+var _ Interface = Empty{}
+
 // Leases implements the [Interface] interface for Empty.
 func (Empty) Leases() (leases []*Lease) { return nil }