Pull request: 3835 check ports properly

Merge in DNS/adguard-home from 3835-imp-error-msg to master

Updates #3835.

Squashed commit of the following:

commit ba31cb67833df9f293fe13be96a35c2a823f115b
Merge: 19c7dfc9 4be69d35
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 16 20:07:25 2021 +0300

    Merge branch 'master' into 3835-imp-error-msg

commit 19c7dfc96284a271d30d7111c86c439be3461389
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 16 19:42:10 2021 +0300

    all: imp more

commit 5b9c6a3e357238bf44ef800a6033a7671f27d469
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 16 18:57:02 2021 +0300

    all: introduce aghhttp

commit 29caa17200957aad2b98461573bb33d80931adcf
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Dec 16 14:23:53 2021 +0300

    all: imp more

commit 754c020191d7b9518cb0e789f3f5741ba38c3cf4
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Dec 15 20:53:41 2021 +0300

    all: imp code, log changes

commit ec712dd562f31fcc2fbc27e7035f926c79827444
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Dec 15 18:40:54 2021 +0300

    home: check ports properly

internal/home/portsmap.go

@@ -0,0 +1,63 @@
+package home
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+
+	"github.com/AdguardTeam/golibs/errors"
+	"github.com/AdguardTeam/golibs/stringutil"
+)
+
+// portsMap is a helper type for mapping a network port to the number of its
+// users.
+type portsMap map[int]int
+
+// add binds each of ps.  Zeroes are skipped.
+func (pm portsMap) add(ps ...int) {
+	for _, p := range ps {
+		if p == 0 {
+			continue
+		}
+
+		pm[p]++
+	}
+}
+
+// validate returns an error about all the ports bound several times.
+func (pm portsMap) validate() (err error) {
+	overbound := []int{}
+	for p, num := range pm {
+		if num > 1 {
+			overbound = append(overbound, p)
+			pm[p] = 1
+		}
+	}
+
+	switch len(overbound) {
+	case 0:
+		return nil
+	case 1:
+		return fmt.Errorf("port %d is already used", overbound[0])
+	default:
+		b := &strings.Builder{}
+
+		// TODO(e.burkov, a.garipov):  Add JoinToBuilder helper to stringutil.
+		stringutil.WriteToBuilder(b, "ports ", strconv.Itoa(overbound[0]))
+		for _, p := range overbound[1:] {
+			stringutil.WriteToBuilder(b, ", ", strconv.Itoa(p))
+		}
+		stringutil.WriteToBuilder(b, " are already used")
+
+		return errors.Error(b.String())
+	}
+}
+
+// validatePorts is a helper function for a single-step ports binding
+// validation.
+func validatePorts(ps ...int) (err error) {
+	pm := portsMap{}
+	pm.add(ps...)
+
+	return pm.validate()
+}