Bug 2214075

There is incorrect optimization in the code that relies on the fact that the TIOCMGET ioctl cannot get value 0 in the arg. It seems this may happen e.g. with the tty0tty null modem emulator, where the "arg == old_status" check is always true (because old_status is explicitly initialized to 0) causing statserial to loops even with the "-n" argument. I will PR workaround patch.

Reproducible: Always

Steps to Reproduce:
1. modprobe tty0tty
2. statserial -n /dev/tnt0
3.
Actual Results:  
It loops until serial status is changed.

Expected Results:  
It returns the status of the serial port.

Incorrect code:
  unsigned int old_status = 0;  /* value of previous call */
  unsigned int arg;             /* value returned by ioctl */
...
  for (;;) {
    /* get modem status info */
    status = ioctl(fd, TIOCMGET, &arg);
...
    /* avoid unneccessary screen updates */
    if (arg == old_status)
    {
      sleep(1);
      continue;
    }
    old_status = arg;
...
For "arg" being "0" we have the loop.

Strace:
openat(AT_FDCWD, "/dev/tnt1", O_RDONLY) = 3
ioctl(3, TIOCMGET, [0])                 = 0
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffc316de900) = 0
ioctl(3, TIOCMGET, [0])                 = 0
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffc316de900) = 0
ioctl(3, TIOCMGET, [0])                 = 0
clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffc316de900) = 0
...