Added new API route to obtain available ports for a device

Call:

curl -H "X-Auth-Token: APITOKEN" http://librenms.org/api/v0/devices/localhost/ports

curl -H "X-Auth-Token: APITOKEN" http://librenms.org/api/v0/devices/localhost/ports?columns=port_id,ifDescr,ifName,ifOperStatus,ifAdminStatus

Output:

{
    "status": "ok",
    "err-msg": "",
    "count": 5,
    "ports": [
        {
            "ifName": "lo"
        },
        {
            "ifName": "eth0"
        },
        {
            "ifName": "eth1"
        },
        {
            "ifName": "eth2"
        },
        {
            "ifName": "eth3"
        }
    ]
}

{
    "status": "ok",
    "err-msg": "",
    "count": 5,
    "ports": [
        {
            "port_id": "126",
            "ifDescr": "lo",
            "ifName": "lo",
            "ifOperStatus": "up",
            "ifAdminStatus": "up"
        },
        {
            "port_id": "127",
            "ifDescr": "eth0",
            "ifName": "eth0",
            "ifOperStatus": "up",
            "ifAdminStatus": "up"
        },
        {
            "port_id": "128",
            "ifDescr": "eth1",
            "ifName": "eth1",
            "ifOperStatus": "down",
            "ifAdminStatus": "down"
        },
        {
            "port_id": "129",
            "ifDescr": "eth2",
            "ifName": "eth2",
            "ifOperStatus": "down",
            "ifAdminStatus": "down"
        },
        {
            "port_id": "130",
            "ifDescr": "eth3",
            "ifName": "eth3",
            "ifOperStatus": "up",
            "ifAdminStatus": "up"
        }
    ]
}