Improve op-mode commands for ipoe-server
https://github.com/vyos/vyos-1x/pull/344
vyos@vyos:~$ show ipoe-server sessions ifname | username | calling-sid | ip | rate-limit | type | comp | state | uptime --------+----------+-------------------+------------+------------+------+------+--------+---------- ipoe0 | | 00:50:79:66:68:09 | 100.64.0.2 | | ipoe | | active | 00:00:11 ipoe1 | | 00:50:79:66:68:0a | 100.64.0.3 | | ipoe | | active | 00:00:09