ng_pppoe: introduce new sysctl net.graph.pppoe.lcp_pcp

This fixes incomplete commit 2e547442ab3822d3d7c46a68f152032ef5fe337c

New sysctl allows to mark transmitted PPPoE LCP Control
ethernet frames with needed 3-bit Priority Code Point (PCP) value.
Confirming driver like if_vlan(4) uses the value to fill
IEEE 802.1p class of service field.

This is similar to Cisco IOS "control-packets vlan cos priority"
command.

It helps to avoid premature disconnection of user sessions
due to control frame drops (LCP Echo etc.)
if network infrastructure has a botteleck at a switch
or the xdsl DSLAM.

See also:
https://sourceforge.net/p/mpd/discussion/44692/thread/c7abe70e3a/

Tested by: Klaus Fokuhl at SourceForge
MFC after: 2 weeks