MFC r266538:
Make ng_mppc to not disable the node in case of multiple packet loss.
Quite often it can be just packet reorder, and killing link in such case
is inconvenient.  Add few sysctl's to control that behavior.

PR:		kern/182212
Submitted by:	Eugene Grosbein <egrosbein@rdtc.ru>
Approved by:	re (glebius)