openbsd_network: Some basic additions in preparation for "rtl8125".

These are trivial and do not really need testing.

openbsd_network: Add a real implementation of MCLGETL.

The old alias had a number of shortcomings which made it not match
OpenBSD and thus special-casing had to be used in drivers. The new
version should match OpenBSD's semantics precisely.

openbsd_network: Correct return values of m_dup_pkthdr and m_tag_copy_chain.

FreeBSD's versions return 1 on success, OpenBSD's return 0.
Fixes at least some of the packet loss in OpenBSD WiFi drivers.

openbsd_network: Move mbuf_list and mbuf_queue into mbuf-obsd.h.

openbsd_network: Implement two mbuf macros in terms of FreeBSD ones.

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.)