kbd: remove kbdsw, store pointer to driver in each keyboard_t
The previous implementation relied on a kbdsw array that mirrored the global
keyboards array. This is fine, but also requires extra locking consideration
when accessing to ensure that it's not being resized as new keyboards are
added.
The extra pointer costs little in a struct that there are relatively few of
on any given system, and simplifies locking requirements ever-so-slightly as
we only need to consider the locking requirements of whichever method is
being invoked.
__FreeBSD_version is bumped as any kbd modules will need rebuilt following
this change.
UnifiedSplitRaw
