dhcp: T7052: Fix remaining time evaluation and formatting errors

[indrajitr]

Change summary

The remaining time for a lease was not being correctly evaluated and formatted. As a result, expired leases show up with show dhcp server leases.

Also, the empty hostname should be replaced by '-'.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

https://vyos.dev/T7052

Related PR(s)

How to test / Smoketest result

Before:

10.10.2.1 has lease expired but still shows up, 10.10.2.2 doesn't have hostname set to -

vyos@test-0125:~$ show dhcp server leases sort ip
IP Address    MAC address        State    Lease start                Lease expiration           Remaining    Pool    Hostname                     Origin
------------  -----------------  -------  -------------------------  -------------------------  -----------  ------  ---------------------------  --------
10.10.1.9     80:aa:bb:cc:dd:ee  active   2025-01-26 08:25:54+00:00  2025-01-27 08:25:54+00:00  13:40:25     LAN1    ngap                         local
10.10.2.1     7e:aa:bb:cc:dd:ee  expired  2025-01-25 17:46:19+00:00  2025-01-26 17:46:19+00:00  -            LAN2    watch                        local
10.10.2.2     a6:aa:bb:cc:dd:ee  active   2025-01-25 23:23:18+00:00  2025-01-26 23:23:18+00:00  4:37:49      LAN2                                 local

After:

10.10.2.1 doesn't show up, 10.10.2.2 has hostname set to - correctly

vyos@test-0125:~$ show dhcp server leases sort ip
IP Address    MAC address        State    Lease start                Lease expiration           Remaining    Pool    Hostname                     Origin
------------  -----------------  -------  -------------------------  -------------------------  -----------  ------  ---------------------------  --------
10.10.1.9     80:aa:bb:cc:dd:ee  active   2025-01-26 08:25:54+00:00  2025-01-27 08:25:54+00:00  13:15:04     LAN1    ngap                         local
10.10.2.2     a6:aa:bb:cc:dd:ee  active   2025-01-25 23:23:18+00:00  2025-01-26 23:23:18+00:00  4:12:28      LAN2    -                            local

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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• CLI Smoketests (no interfaces) 👍 passed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
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• TPM tests 👍 passed