T3355: always work with UNIX timestamps for dhcp client lease display

ISC dhcp client contains least_update timestamp in human readable format this
makes less sense for an API and also the expiry timestamp is provided in UNIX
time. Convert string (e.g. Sun Jul 30 18:13:44 CEST 2023) to UNIX time (1690733624)

vyos@vyos:~$ ${vyos_op_scripts_dir}/dhcp.py show_client_leases --family inet --interface eth0.10
Interface    eth0.10
IP address   172.16.33.123                  [Active]
Subnet Mask  255.255.255.0
Domain Name  vyos.net
Router       172.16.33.254
Name Server  172.16.254.30
DHCP Server  172.16.33.254
DHCP Server  86400
Last Update  Sun Jul 30 18:13:44 CEST 2023
Expiry       Mon Jul 31 18:13:43 CEST 2023

vyos@vyos:~$ ${vyos_op_scripts_dir}/dhcp.py show_client_leases --family inet --interface eth0.10 --raw
[
    {
        "last_update": 1690733624,
        "reason": "RENEW",
        "interface": "eth0.10",
        "new_expiry": "1690820023",
        "new_dhcp_lease_time": "86400",
        "medium": "",
        "alias_ip_address": "",
        "new_ip_address": "172.16.33.123",
        "new_broadcast_address": "172.16.33.255",
        "new_subnet_mask": "255.255.255.0",
        "new_domain_name": "vyos.net",
        "new_network_number": "172.16.33.0",
        "new_domain_name_servers": "172.16.254.30",
        "new_routers": "172.16.33.254",
        "new_static_routes": "",
        "new_dhcp_server_identifier": "172.16.33.254",
        "new_dhcp_message_type": "5",
        "old_ip_address": "172.16.33.123",
        "old_subnet_mask": "255.255.255.0",
        "old_domain_name": "vyos.net",
        "old_domain_name_servers": "172.16.254.30",
        "old_routers": "172.16.33.254",
        "old_static_routes": ""
    }
]

0 files changed, 0 insertions, 0 deletions