openbsd_network & drivers: Implement the real bus_dmamem functions.

This required the addition of some slight modifications to the main
(FreeBSD compat) bus_dma implementation.

Allows some #ifdefs from iaxwifi/idualwifi to be dropped, and will
ease the porting of some future drivers.

Tested with idualwifi7260, seems to work as before.

openbsd_network: Use the new bus_dmamap_sync_etc.

This preserves the semantics of partial syncs. Unfortunately in my
testing it does not seem to improve the packet loss I was seeing...

openbsd_network: Adjust reference-taking in bus_dmamap_load_mbuf.

dm_segs is a flexible-length array at the end of a structure.
Taking its address with & gives the same address as using it without,
however, the types are not identical in the different cases.

This should fix some warnings about incompatibile pointer types
while not changing behavior.

OpenBSD-to-FreeBSD/Haiku compatibility headers.

OpenBSD's and FreeBSD's kernel APIs are very similar, but there are
still some differences. Fortunately these are small enough that we
can take care of most of them with a header-only shim layer.

(The files and functions implemented by this commit were implemented
and tested against the modules and drivers imported and adapted in
the following commits. Some of this took quite a bit of time to get
right, especially where the APIs have diverged in very subtle ways.)