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/nspawn/nspawn.c

@@ -725,7 +725,7 @@ static int parse_argv(int argc, char *argv[]) {
                 case ARG_NETWORK_ZONE: {
                         char *j;
 
-                        j = strappend("vz-", optarg);
+                        j = strjoin("vz-", optarg);
                         if (!j)
                                 return log_oom();