On sun4/sun4c with a virtual address hole (starting at 512MB), this seems
like a reasonable compromise: about 196MB left for shared libs and sysv-style
shared memory segments.
On the sun4m the limits can easily be made larger: consider turning
MAXTSIZ and MAXDSIZ into tunable variables..
cpu.h: cpu_adjstack() won't work well in the NetBSD execve - need a
different model, stub enablertclock() for now..
exec.h: a netbsd-type exec.h file (ie. empty)
param.h: add MID_MACHINE
pmap.h: delete kernel_pmap hack - it doesn't work for us, + two proto's
vmparam.h: VM_MIN_ADDRESS = 0!