This is the logical merge of rev 1.32 of i4b's old if_spppsubr.c (which
was based on PR misc/11767), plus (i4b) rev 1.6 of i4b's if_ispppsubr.c,
albeit with numerous stylistic and cosmetic changes.
PR: misc/11767
Submitted by: i4b, Joachim Kuebart
MFC after: 1 month