lightly used. Find the proper .m file when we depend on *_if.[ch] in
the srcs line, with seat-belts for false positive matches. This uses
make's path mechanism. A further refinement would be to calculate this
once, and then pass the resulting _MPATH to modules submakes.
Differential Revision: https://reviews.freebsd.org/D2327
Support for the multiport feature is mostly implemented, but currently
disabled due to some potential races in the hot plug code paths.
Requested by: marcel
MFC after: 1 month
Relnotes: yes