The existing code allocated 2 bytes (char*), but gtty() needs 6 bytes
(struct sgttyb), so isatty() smashed the stack and corrupted its
return address, probably causing SIGBUS or SIGSEGV.
Fix by switching to TIOCGETD, which needs 2 bytes. TIOCGETD isn't in
the manual for tty(4), but does appear in
https://minnie.tuhs.org//cgi-bin/utree.pl?file=V7/usr/sys/dev/tty.c
This fixes hilo_c.pdpv7 and hilo_mod.pdpv7 in simh-pdp11.
