Pull request:* all: fix all staticcheck simplification and unused warnings

Merge in DNS/adguard-home from 2270-fix-s-u-warnings to master

Squashed commit of the following:

commit 03e0f78bd471057007c2d4042ee26eda2bbc9b29
Merge: 50dc3ef5c 7e16fda57
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Nov 6 11:54:09 2020 +0300

    Merge branch 'master' into 2270-fix-s-u-warnings

commit 50dc3ef5c44a5fdc941794c26784b0c44d7b5aa0
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Nov 5 19:48:54 2020 +0300

    * all: improve code quality

commit d6d804f759ce3e47154a389b427550e72c4b9090
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Thu Nov 5 18:03:35 2020 +0300

    * all: fix all staticcheck simplification and unused warnings

    Closes #2270.

internal/dhcpd/nclient4/client.go

@@ -48,14 +48,12 @@ const (
 	ServerPort = 67
 )
 
-var (
-	// DefaultServers is the address of all link-local DHCP servers and
-	// relay agents.
-	DefaultServers = &net.UDPAddr{
-		IP:   net.IPv4bcast,
-		Port: ServerPort,
-	}
-)
+// DefaultServers is the address of all link-local DHCP servers and
+// relay agents.
+var DefaultServers = &net.UDPAddr{
+	IP:   net.IPv4bcast,
+	Port: ServerPort,
+}
 
 var (
 	// ErrNoResponse is returned when no response packet is received.
@@ -375,14 +373,6 @@ func WithHWAddr(hwAddr net.HardwareAddr) ClientOpt {
 	}
 }
 
-// nolint
-func withBufferCap(n int) ClientOpt {
-	return func(c *Client) (err error) {
-		c.bufferCap = n
-		return
-	}
-}
-
 // WithRetry configures the number of retransmissions to attempt.
 //
 // Default is 3.

internal/dhcpd/nclient4/client_test.go

@@ -122,6 +122,13 @@ func newPacket(op dhcpv4.OpcodeType, xid dhcpv4.TransactionID) *dhcpv4.DHCPv4 {
 	return p
 }
 
+func withBufferCap(n int) ClientOpt {
+	return func(c *Client) (err error) {
+		c.bufferCap = n
+		return
+	}
+}
+
 func TestSendAndRead(t *testing.T) {
 	for _, tt := range []struct {
 		desc   string