tree-wide: get rid of strappend()

It's a special case of strjoin(), so no need to keep both. In particular
as typing strjoin() is even shoert than strappend().

src/systemctl/systemctl.c

@@ -6459,7 +6459,7 @@ static int enable_sysv_units(const char *verb, char **args) {
                 }
 
                 if (!isempty(arg_root)) {
-                        q = strappend("--root=", arg_root);
+                        q = strjoin("--root=", arg_root);
                         if (!q)
                                 return log_oom();