sgimips/scn(4): Convert to ttylock/ttyunlock.

Remove unused extern declaration of constty.

don't malloc a tiny, fixed size buffer to scribble into, then not use it
and never free it either

found by Brainy

Add d_discard to all struct cdevsw instances I could find.

All have been set to "nodiscard"; some should get a real implementation.

- use cpu_{g,s}etmodel
- fix unused

Change (mostly mechanically) every cdevsw/bdevsw I can find to use
designated initializers.

I have not built every extant kernel so I have probably broken at
least one build; however I've also found

Change (mostly mechanically) every cdevsw/bdevsw I can find to use
designated initializers.

I have not built every extant kernel so I have probably broken at
least one build; however I've also found and fixed some wrong
cdevsw/bdevsw entries so even if so I think we come out ahead.

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split device_t/softc for all remaining drivers.
replace "struct device *" with "device_t".
use device_xname(), device_unit(), etc.

Rename ttymalloc() to tty_alloc(), and ttyfree() to tty_free() for
consistency. Remove some unnecessary malloc.h inclusions as well.

Use lwp_getpcb() on mips, powerpc and sh3, clean from struct user usage.

Check in work in progress of Personal IRIS 4D/20 and 4D/25 support (IP6
and IP10, respectively).