ng_pppoe(8): add support for user-supplied Host-Uniq tag.

A few ISP filter PADI requests based on such tag,
to force the use of their own routers.
The custom Host-Uniq tag is passed in the NGM_PPPOE_CONNECT
control message, so it can be used with FreeBSD ppp(8)
and mpd without any other change.

Add support to send and receive PADM messages,
HURL and MOTM, often used by service providers to provide
ACS information and other configuration settings
to the user CPE.

Submitted by: ale
Approved by: mav (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D9270