replace atoi with strtol (sonic-net#6)

From Semgrep: https://semgrep.dev/r?q=c.lang.correctness.incorrect-use-ato-fn.incorrect-use-ato-fn

> Avoid the 'ato*()' family of functions. Their use can lead to undefined behavior, integer overflows, and lack of appropriate error handling. Instead prefer the 'strtol*()' family of functions.

From atoi() man page: https://www.man7.org/linux/man-pages/man3/atoi.3.html

> The atoi() function converts the initial portion of the string pointed to by nptr to int.  The behavior is the same as
>            strtol(nptr, NULL, 10);
> except that atoi() does not detect errors.

Therefore, replace atoi() with strtol()

Verified by kill dhcpmon process, installing new .deb change to dhcp_relay docker, execute dhcpmon process, check if counters is printing in syslog, compare counters before and after this change.
logs: 
[before.txt](https://github.com/sonic-net/sonic-dhcpmon/files/11377335/before.txt)
[after.txt](https://github.com/sonic-net/sonic-dhcpmon/files/11377336/after.txt)

example commands: 
[command.txt](https://github.com/sonic-net/sonic-dhcpmon/files/11423945/command.txt)

src/main.cpp

@@ -114,11 +114,13 @@ int main(int argc, char **argv)
 {
     int rv = EXIT_FAILURE;
     int i;
+    char *endptr;
     int window_interval = dhcpmon_default_health_check_window;
     int max_unhealthy_count = dhcpmon_default_unhealthy_max_count;
     size_t snaplen = dhcpmon_default_snaplen;
     int make_daemon = 0;
     bool debug_mode = false;
+    errno = 0;
 
     setlogmask(LOG_UPTO(LOG_INFO));
     openlog(basename(argv[0]), LOG_CONS | LOG_PID | LOG_NDELAY, LOG_DAEMON);
@@ -150,15 +152,27 @@ int main(int argc, char **argv)
             i++;
             break;
         case 's':
-            snaplen = atoi(argv[i + 1]);
+            snaplen = strtol(argv[i + 1], &endptr, 10);
+            if (errno != 0 || *endptr != '\0') {
+                fprintf(stderr, "%s: %s: Invalid snap length\n", basename(argv[0]), argv[i + 1]);
+                usage(basename(argv[0]));
+            }
             i += 2;
             break;
         case 'w':
-            window_interval = atoi(argv[i + 1]);
+            window_interval = strtol(argv[i + 1], &endptr, 10);
+            if (errno != 0 || *endptr != '\0') {
+                fprintf(stderr, "%s: %s: Invalid window interval\n", basename(argv[0]), argv[i + 1]);
+                usage(basename(argv[0]));
+            }
             i += 2;
             break;
         case 'c':
-            max_unhealthy_count = atoi(argv[i + 1]);
+            max_unhealthy_count = strtol(argv[i + 1], &endptr, 10);
+            if (errno != 0 || *endptr != '\0') {
+                fprintf(stderr, "%s: %s: Invalid max unhealthy count\n", basename(argv[0]), argv[i + 1]);
+                usage(basename(argv[0]));
+            }
             i += 2;
             break;
         case 'D':