ng_pppoe: MFC: introduce new sysctl net.graph.pppoe.lcp_pcp
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"
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.
Tested by: Klaus Fokuhl at SourceForge