Pull request: 2546 updater fix

Merge in DNS/adguard-home from 2546-updater-fix to master

Closes #2546.

Squashed commit of the following:

commit af243c9fad710efe099506fda281e628c3e5ec30
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jan 13 14:33:37 2021 +0300

    updater: fix go 1.14 compat

commit 742fba24b300ce51c04acb586996c3c75e56ea20
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Wed Jan 13 13:58:27 2021 +0300

    util: imp error check

commit c2bdbce8af657a7f4b7e05c018cfacba86e06753
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Jan 11 18:51:26 2021 +0300

    all: fix and refactor update checking

internal/version/version.go

@@ -0,0 +1,53 @@
+// Package version contains AdGuard Home version information.
+package version
+
+import (
+	"fmt"
+	"runtime"
+)
+
+// These are set by the linker.  Unfortunately we cannot set constants during
+// linking, and Go doesn't have a concept of immutable variables, so to be
+// thorough we have to only export them through getters.
+//
+// TODO(a.garipov): Find out if we can get GOARM and GOMIPS values the same way
+// we can GOARCH and GOOS.
+var (
+	channel string
+	goarm   string
+	gomips  string
+	version string
+)
+
+// Channel returns the current AdGuard Home release channel.
+func Channel() (v string) {
+	return channel
+}
+
+// Full returns the full current version of AdGuard Home.
+func Full() (v string) {
+	msg := "AdGuard Home, version %s, channel %s, arch %s %s"
+	if goarm != "" {
+		msg = msg + " v" + goarm
+	} else if gomips != "" {
+		msg = msg + " " + gomips
+	}
+
+	return fmt.Sprintf(msg, version, channel, runtime.GOOS, runtime.GOARCH)
+}
+
+// GOARM returns the GOARM value used to build the current AdGuard Home release.
+func GOARM() (v string) {
+	return goarm
+}
+
+// GOMIPS returns the GOMIPS value used to build the current AdGuard Home
+// release.
+func GOMIPS() (v string) {
+	return gomips
+}
+
+// Version returns the AdGuard Home build version.
+func Version() (v string) {
+	return version
+}