Not only did these two drivers depend on IFF_NEEDSGIANT, they were
broken 7 months ago during the MPSAFE TTY import. if_ppp(4) has been
replaced by ppp(8). There is no replacement for if_sl(4).
If we see regressions in for example the ports tree, we should just use
__FreeBSD_version 800045 to check whether if_ppp(4) and if_sl(4) are
present. Version 800045 is used to denote the import of MPSAFE TTY.
Discussed with: rwatson, but also rwatson's IFF_NEEDSGIANT emails on the
lists.
