Pull request: all: rm var shadowing vol. 3

Updates #2803.

Squashed commit of the following:

commit ce711ae2e08c3714a4e0c6d177c02f7801c05fce
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:58:40 2021 +0300

    all: imp code, docs

commit e91094e461893c27eda879b33b5ed2c7085510df
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Mar 11 20:42:14 2021 +0300

    all: rm var shadowing vol. 3

internal/dhcpd/http.go

@@ -282,7 +282,9 @@ func (s *Server) handleDHCPInterfaces(w http.ResponseWriter, r *http.Request) {
 			// this interface doesn't support broadcast, skip it
 			continue
 		}
-		addrs, err := iface.Addrs()
+
+		var addrs []net.Addr
+		addrs, err = iface.Addrs()
 		if err != nil {
 			httpError(r, w, http.StatusInternalServerError, "Failed to get addresses for interface %s: %s", iface.Name, err)
 			return