limit the number of FDs to select on to the maximum supported by NetBSD --
which is as many FDs as the emul_ultrix process can have open, anyway.
Add emulation of Ultrix getmnt(2) in ultrix_fs.c
Add partial emulation of Ultrix tty ioctl()s in ultrix_ioctl.c, derived
from compat/sunos/sunos_ioctl.c. Ultrix libc's ``isatty()'' now works
in compat_emul processes.