bus: Provide 32-bit compatibility

[des]

Summary

Implement 32-bit compatibility for hw.bus.devices and hw.bus.rman.
This allows a 32-bit devinfo(8) to work on a 64-bit kernel.

[kib]

sys/kern/subr_bus.c

5589

Why ignoring the error?
I believe even SBUF_FIXEDLEN would get into an error state on overflow of the buffer.

sys/kern/subr_rman.c

1031

IMO it is worth extracting the same code into a small helper function and calling it twice, instead of copying.

1052

There should be a blank line after the local variables block. There and in several other places in the patch.