MFC r298931, r298981, r299375:

Minor spelling fixes in:
sys/dev, sys/sys

Many of these have user-visible strings.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Mechanically substitute flags from historic mbuf allocator with
malloc(9) flags in sys/dev.

Introduce wtap, the beginnings of a net80211 wlan simulator.

This introduces:

* a basic wtap interface
* a HAL, which implements an abstraction layer for implementing
different device behavious;
* A visibility plugin, which allows for control over which nodes
see other nodes (useful for mesh work.)

It doesn't yet implement sta/adhoc/hostap modes but these are quite
feasible to implement.

Monthadar uses it to do 802.11s mesh verification.

The userland tools will be committed in a follow-up commit.

Submitted by: Monthadar Al Jaberi <monthadar@gmail.com>