(REGISTER_U_ADDR): Fix up for Ultrix 4.2.
* tm-tahoe.h, xm-tahoe.h (KERNEL_U_ADDR): Move macro to xm-tahoe.h.
* stack.c (_initialize_stack): "dow" should be a valid abbreviation
for "down". Suggested by Richard Stallman.
cross-debugging and not necessary anymore (see IN_SIGTRAMP in
xm-vax.h)).
[SET_STACK_LIMIT_HUGE]: Include <sys/{resource,time}.h>.
xm-tahoe.h: Don't define _DIRENT_.
* xm-tahoe.h: Remove USE_OLD_TTY (not needed now that terminal.h
includes sgtty.h before sys/ioctl.h).