T5779: conntrack: Apply fixes to <set system conntrack timeout custom> (backport #2574)

[mergify]

This is an automatic backport of pull request #2574 done by Mergify.

Backport of the conntrack system from current branch.

(cherry picked from commit fd0bcaf)
(cherry picked from commit 5acf5ac)
(cherry picked from commit 42ff4d8)
(cherry picked from commit 24a1a70)

cpo@LR2.wue3:~$ /usr/libexec/vyos/tests/smoke/cli/test_system_conntrack.py
test_conntrack_hash_size (__main__.TestSystemConntrack.test_conntrack_hash_size) ... ok
test_conntrack_ignore (__main__.TestSystemConntrack.test_conntrack_ignore) ... ok
test_conntrack_module_enable (__main__.TestSystemConntrack.test_conntrack_module_enable) ... ok
test_conntrack_options (__main__.TestSystemConntrack.test_conntrack_options) ... ok
test_conntrack_timeout_custom (__main__.TestSystemConntrack.test_conntrack_timeout_custom) ... ok

----------------------------------------------------------------------
Ran 5 tests in 46.411s

OK

Full make test and make testc also pass

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[c-po]

@nicolas-fort code-base has diverged on this and cherry-picking is not easily possible. Can you please provide a manual backport?