striving to keep the diffs short and simple.
* Replace FreeBSDisms (e.g. struct arpcom) with conditionally-compiled
NetBSDism (struct ethercom).
* Add compatibility shims in ieee80211_compat.*: provide NetBSD with
if_printf, for example.
* Convert FreeBSD node mutex uses to generic node critical-section
protection (ieee80211_node_critsect_begin, _end), replace
FreeBSD atomic arithmetic with generic alternative, and implement
generics in NetBSD
* Provide NetBSD-style 802.11 ioctls
* Style nits