Pull request: 2179 ipset subdomains

Merge in DNS/adguard-home from 2179-ipset-subdomains to master

Closes #2179.

Squashed commit of the following:

commit de17caac4c2ea2bc7931f162c6dfa7822a71554f
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Fri Jan 29 18:34:46 2021 +0300

    dnsforward: imp code, docs

commit e5ab957560bcfba80feac4b72f9b22535ecd4c7d
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Jan 26 20:43:31 2021 +0300

    dnsforward: imp code

commit 2b84d27b752832885e4896d0e75de2576e2b965b
Author: David Sheets <sheets@alum.mit.edu>
Date:   Tue Oct 6 16:34:06 2020 +0100

    dnsforward: support subdomain matching in ipset

    This is a squash of all commits in #2179.

internal/dnsforward/ipset_others.go

@@ -0,0 +1,26 @@
+// +build !linux
+
+package dnsforward
+
+import (
+	"github.com/AdguardTeam/golibs/log"
+)
+
+type ipsetCtx struct{}
+
+// init initializes the ipset context.
+func (c *ipsetCtx) init(ipsetConfig []string) (err error) {
+	if len(ipsetConfig) != 0 {
+		log.Info("ipset: only available on linux")
+	}
+
+	return nil
+}
+
+// process adds the resolved IP addresses to the domain's ipsets, if any.
+func (c *ipsetCtx) process(_ *dnsContext) (rc resultCode) {
+	return resultCodeSuccess
+}
+
+// Close closes the Linux Netfilter connections.
+func (c *ipsetCtx) Close() (_ error) { return nil }