For easier debugging add additional subsystems to the CLI for `show log` and `monitor log`
## Add
### show log
* `show log pppoe`
* `show log pppoe interface <interface>`
### monitor log
* `monitor log pppoe`
* `monitor log pppoe interface <interface>`
* `monitor log dhcp`
* `monitor log dhcp client interface <interface>`
* `monitor log dhcpv6`
* `monitor log dhcpv6 client interface <interface>`