Pull request: all: fix lint and naming issues vol. 2

Merge in DNS/adguard-home from 2276-fix-lint-2 to master

Updates #2276.

Squashed commit of the following:

commit 24760b9586bb31be134ef9518dbece485560b1a0
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Dec 7 14:39:50 2020 +0300

    all: fix lint and naming issues vol. 2

internal/dhcpd/dhcpd.go

@@ -52,6 +52,7 @@ type ServerConfig struct {
 	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request)) `yaml:"-"`
 }
 
+// OnLeaseChangedT is a callback for lease changes.
 type OnLeaseChangedT func(flags int)
 
 // flags for onLeaseChanged()
@@ -74,6 +75,7 @@ type Server struct {
 	onLeaseChanged []OnLeaseChangedT
 }
 
+// ServerInterface is an interface for servers.
 type ServerInterface interface {
 	Leases(flags int) []Lease
 	SetOnLeaseChanged(onLeaseChanged OnLeaseChangedT)