Read getent's exit code, not its wording (novox/hq issue 213)

A user that does not exist yet was matched as Go's 'exit status 2', while the host's runner says
'getent exited 2', so it read as a user database that did not answer: the controller's account was
never created and the handover to its process stopped there. The runner keeps the exit underneath
its words, and a caller asks the code.

internal/system/system.go

@@ -19,6 +19,9 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"os/exec"
+	"regexp"
+	"strconv"
 	"strings"
 
 	"github.com/novox/mesh-host/internal/declaration"
@@ -95,8 +98,10 @@ func LookUpUser(ctx context.Context, run Runner, name string) (Login, bool, erro
 	out, err := run(ctx, "getent", "passwd", name)
 	if err != nil {
 		// getent's own convention: 2 means the key was not found, which is the only failure that
-		// means "no such user".
-		if strings.Contains(err.Error(), "exit status 2") {
+		// means "no such user". Asked of the exit code: matched as text, it looked for Go's wording
+		// ("exit status 2") while the host's runner says "getent exited 2", so a user that did not
+		// exist yet read as a database that did not answer, and no account was ever created.
+		if code, ok := ExitCode(err); ok && code == 2 {
 			return Login{}, false, nil
 		}
 		return Login{}, false, fmt.Errorf(
@@ -221,3 +226,27 @@ func For(name string) (System, error) {
 func All() []System {
 	return []System{arch{}, alpine{}, android{}}
 }
+
+// ExitCode is the code a command exited with, when err says one: from the exit itself where the
+// runner kept it, else from the words either runner shape uses ("exit status N", "<cmd> exited N").
+func ExitCode(err error) (int, bool) {
+	if err == nil {
+		return 0, false
+	}
+	var exit *exec.ExitError
+	if errors.As(err, &exit) {
+		return exit.ExitCode(), true
+	}
+	if m := exitWords.FindStringSubmatch(err.Error()); len(m) == 3 {
+		for _, g := range m[1:] {
+			if g != "" {
+				if n, convErr := strconv.Atoi(g); convErr == nil {
+					return n, true
+				}
+			}
+		}
+	}
+	return 0, false
+}
+
+var exitWords = regexp.MustCompile(`exit status (\d+)|exited (\d+)`)