Pull request 1989: AG-24794-client-source

Squashed commit of the following:

commit 5cf83aafc8ddfea15b4f8e9b4061af021727f68c
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Fri Sep 1 14:58:03 2023 +0300

    client: imp code

commit 4325c8f610d1329ad38c800e20275668f1690f51
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Fri Sep 1 14:38:33 2023 +0300

    all: add client source

internal/arpdb/arpdb.go

@@ -77,8 +77,8 @@ func (n Neighbor) Clone() (clone Neighbor) {
 	}
 }
 
-// validatedHostname returns valid hostname.  Otherwise returns empty string and
-// logs the error if hostname is not valid.
+// validatedHostname returns h if it's a valid hostname, or an empty string
+// otherwise, logging the validation error.
 func validatedHostname(h string) (host string) {
 	err := netutil.ValidateHostname(h)
 	if err != nil {