greeter: Add support for Debian greetd user/group name (#1685)

* greeter: Detect user and group used by greetd

On most distros greetd runs as user and group "greeter",
but on Debian the user and group "_greetd" are used.

* greeter: Use correct group in sync command

* greeter: more generic group detection

---------

Co-authored-by: bbedward <bbedward@gmail.com>

core/internal/utils/group.go

@@ -0,0 +1,37 @@
+package utils
+
+import (
+	"os"
+	"strings"
+)
+
+func HasGroup(groupName string) bool {
+	return HasGroupIn(groupName, "/etc/group")
+}
+
+func HasGroupIn(groupName, path string) bool {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return false
+	}
+	return HasGroupData(groupName, string(data))
+}
+
+func HasGroupData(groupName, data string) bool {
+	prefix := groupName + ":"
+	for line := range strings.SplitSeq(data, "\n") {
+		if strings.HasPrefix(line, prefix) {
+			return true
+		}
+	}
+	return false
+}
+
+func FindGroupData(data string, candidates ...string) (string, bool) {
+	for _, candidate := range candidates {
+		if HasGroupData(candidate, data) {
+			return candidate, true
+		}
+	}
+	return "", false
+}