Fix MPD5 PPPoE support on vlans and add basic parsing of PADM messages

[ale]

Summary

The netgraph node paths cannot contain '.' and ':' characters, so the ng_ether module sanitizes interface names containing them (like the new format for vlans, eg. fxp0.1) by replacing illegal characters with underscores. This patch keeps interface name and netgraph path separated in the pppoe layer so that mpd can be used successfully on vlan interfaces.

iface = fxp0.1
path = fxp0_1:

Moreover I've added basic support to display PADM messages, introduced by the following patch: https://reviews.freebsd.org/D9270

Test Plan

I've setup pppoe connections on vlans and used ngctl to test the PADM messages.

[mav]

I have no objections.

[ale]

Do you plan the apply the patch upstream and release a new version or may I apply it directly to the ports tree?

[julian]

looks ok.
Who;s going to commit?
any doc change required?

[ale]

No need to change documentation IMO, the manual already states that you have to specify the ethernet interface name:

set pppoe iface interface-name

Set the name of the Ethernet interface to use. PPPoE does not affect normal use of the interface.

[ale]

Hey @eugen_grosbein.net since you are the new maintainer of the mpd5 port can you commit also this patch, please? Thanks