Add 'self-update' command

This commit adds a command called `self-update` which downloads the
latest released version of restic from GitHub and replacing the current
binary with it. It does not rely on any external program (so it'll work
everywhere), but still verifies the GPG signature using the embedded GPG
public key.

By default, the `self-update` command is hidden behind the `selfupdate`
built tag, which is only set when restic is built using `build.go`. The
reason for this is that downstream distributions will then not include
the command by default, so users are encouraged to use the
platform-specific distribution mechanism.

build.go

@@ -46,9 +46,10 @@ import (
 
 // config contains the configuration for the program to build.
 var config = Config{
-	Name:      "restic",                              // name of the program executable and directory
-	Namespace: "github.com/restic/restic",            // subdir of GOPATH, e.g. "github.com/foo/bar"
-	Main:      "github.com/restic/restic/cmd/restic", // package name for the main package
+	Name:             "restic",                              // name of the program executable and directory
+	Namespace:        "github.com/restic/restic",            // subdir of GOPATH, e.g. "github.com/foo/bar"
+	Main:             "github.com/restic/restic/cmd/restic", // package name for the main package
+	DefaultBuildTags: []string{"selfupdate"},                // specify build tags which are always used
 	Tests: []string{ // tests to run
 		"github.com/restic/restic/internal/...",
 		"github.com/restic/restic/cmd/...",
@@ -58,11 +59,12 @@ var config = Config{
 
 // Config configures the build.
 type Config struct {
-	Name       string
-	Namespace  string
-	Main       string
-	Tests      []string
-	MinVersion GoVersion
+	Name             string
+	Namespace        string
+	Main             string
+	DefaultBuildTags []string
+	Tests            []string
+	MinVersion       GoVersion
 }
 
 var (
@@ -461,7 +463,7 @@ func main() {
 		os.Exit(1)
 	}
 
-	buildTags := []string{}
+	buildTags := config.DefaultBuildTags
 
 	skipNext := false
 	params := os.Args[1:]
@@ -490,7 +492,7 @@ func main() {
 				die("-t given but no tag specified")
 			}
 			skipNext = true
-			buildTags = strings.Split(params[i+1], " ")
+			buildTags = append(buildTags, strings.Split(params[i+1], " ")...)
 		case "-o", "--output":
 			skipNext = true
 			outputFilename = params[i+1]