qrexec: exit with code 126 when service request was refused

Exit code 1 is very common in all kind of programs, including qrexec
services, so it is hard to distinguish remote failure from service call
refusal. Use something from top of the range here (but not 127, as it is
commonly used to report "Command not found")

QubesOS/qubes-issues#2861

(cherry picked from commit d2aa21625c)

qrexec/qrexec-client-vm.c

@@ -109,7 +109,7 @@ int main(int argc, char **argv)
     ret = read(trigger_fd, &exec_params, sizeof(exec_params));
     if (ret == 0) {
         fprintf(stderr, "Request refused\n");
-        exit(1);
+        exit(126);
     }
     if (ret < 0 || ret != sizeof(exec_params)) {
         perror("read");