autoloading compat43 on tty ioctls

[nia <nia%NetBSD.org@localhost>]

syzkaller found a bug in tty_43.c which i noticed could lock up my
system after i wrote a reproducer for it.

i was curious why 43bsd compat is enabled by default on amd64 then
noticed it gets loaded by tty if it detects an unrecognized ioctl

https://github.com/NetBSD/src/blob/trunk/sys/kern/tty.c#L1420

this seems to be a strange choice, especially on archs that never ran
43bsd binaries. were applications relying on these ioctls more recently?

i'm not familiar with the history of this code, but maybe someone here is