[S6100] Improve S6100 serial-getty monitor, wait and re-check when getty not running to avoid false alert.
#### Why I did it
On S6100, the serial-getty service some time can't auto-restart by systemd. So there is a monit unit to check serial-getty service status and restart it.
However, this monit will report false alert, because in most case when serial-getty not running, systemd can restart it successfully.
To avoid the false alert, improve the monitor to wait and re-check.
Steps to reproduce this issue:
1. User login to device via console, and keep the connection.
2. User login to device via SSH, check the serial-getty@ttyS1.service service, it's running.
3. Run 'monit reload' from SSH connection.
4. Check syslog 1 minutes later, there will be false alert: ' 'serial-getty' process is not running'
#### How I did it
Add check-getty.sh script to recheck again later when getty service not running.
And update monit unit to check serial-getty service status with this script to avoid false alert.
#### How to verify it
Pass all UT.
Manually check fixed code work correctly:
```
admin@***:~$ sudo systemctl stop serial-getty@ttyS1.service
admin@***:~$ sudo /usr/local/bin/check-getty.sh
admin@***:~$ echo $?
1
admin@***:~$ sudo systemctl status serial-getty@ttyS1.service
● serial-getty@ttyS1.service - Serial Getty on ttyS1
Loaded: loaded (/lib/systemd/system/serial-getty@.service; enabled-runtime; vendor preset: enabled)
Active: inactive (dead) since Tue 2023-03-28 07:15:21 UTC; 1min 13s ago
admin@***:~$ sudo /usr/local/bin/check-getty.sh
admin@***:~$ echo $?
0
admin@***:~$ sudo systemctl status serial-getty@ttyS1.service
● serial-getty@ttyS1.service - Serial Getty on ttyS1
Loaded: loaded (/lib/systemd/system/serial-getty@.service; enabled-runtime; vendor preset: enabled)
```
syslog:
```
Mar 28 07:10:37.597458 *** INFO systemd[1]: serial-getty@ttyS1.service: Succeeded.
Mar 28 07:12:43.010550 *** ERR monit[593]: 'serial-getty' status failed (1) -- no output
Mar 28 07:12:43.010744 *** INFO monit[593]: 'serial-getty' trying to restart
Mar 28 07:12:43.010846 *** INFO monit[593]: 'serial-getty' stop: '/bin/systemctl stop serial-getty@ttyS1.service'
Mar 28 07:12:43.132172 *** INFO monit[593]: 'serial-getty' start: '/bin/systemctl start serial-getty@ttyS1.service'
Mar 28 07:13:43.286276 *** INFO monit[593]: 'serial-getty' status succeeded (0) -- no output
```
#### Description for the changelog
[S6100] Improve S6100 serial-getty monitor.
#### Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.
