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
It exposes I/O resources to user space, so that programs can peek
and poke at the hardware. It does not itself have knowledge about
the hardware device it attaches to.
Sponsored by: Juniper Networks, Inc.
