This fixes long standing kernel crashes (MMU fault, address error,
and silent freeze by a double bus fault etc. seen for ~10 years)
caused by kernel stack overflow, especially on x68k and sun3 running
Xorg based servers. See PR/55990 for more details.
"This change seems perfectly reasonable" from thorpej@ and
jklos@ also reported this also solved freeze of his mac68k system
with 10 megabyes of memory.
Should be pulled up to netbsd-9 and netbsd-8.
1069d0beca