Restart pmon in syncd.sh to clean up invalid fd

[ganglyu]

Why I did it

pmon holds file descriptors that become invalid when syncd restarts, causing pmon to malfunction. Stopping pmon before syncd starts/stops ensures pmon always restarts with clean state after syncd is ready.

Work item tracking

• Microsoft ADO (number only):

How I did it

1. In syncd.sh startplatform(): stop pmon before syncd starts (non-warm-boot only), so pmon restarts cleanly after syncd is ready and invalid fds are cleared.
2. In syncd.sh stopplatform1(): stop pmon ahead of syncd on Mellanox cold reboot, ensuring pmon does not hold stale fds during the shutdown sequence.
3. In pmon.service.j2: added After=syncd.service for Mellanox, so systemd starts pmon only after syncd is up.

How to verify it

On a Mellanox device, perform a cold reboot and confirm:

• pmon stops before syncd during shutdown
• pmon starts after syncd comes up
• No invalid fd errors in syslog after reboot

Which release branch to backport (provide reason below if selected)

• 202305
• 202311
• 202405
• 202411
• 202505
• 202511

Tested branch (Please provide the tested image version)

Description for the changelog

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)

[mssonicbld]

/azp run Azure.sonic-buildimage

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