pimd: fix memory leaks in parent process after fork()

[jaredmauch]

Fix memory leaks that occur when the parent process exits after fork() in frr_daemonize(). The parent process was not cleaning up memory allocated before fork(), specifically:

• router structure allocated in pim_router_init()
• MLAG resources (mlag_fifo and mlag_stream) allocated in pim_mlag_init()
• zclient structures (pim_zclient and pim_zlookup) allocated in pim_zebra_init()

Add a new frr_parent_cleanup hook that is called in the parent process before it exits. Register pim_parent_cleanup() to free all PIM-specific resources in the parent process.

Also add NULL pointer checks in pim_mlag_terminate() to safely handle cases where MLAG was not initialized (e.g., pim6d).

Related PR: #18718

[mjstapp]

No "merge" commits, please. Just rebase your commit to newer master as needed.

[ton31337]

@Mergifyio backport stable/10.5 stable/10.4 stable/10.3 stable/10.2

[mergify[bot]]

backport stable/10.5 stable/10.4 stable/10.3 stable/10.2

🟠 Waiting for conditions to match

• [ ] merged [📌 backport requirement]