Prevent crash in isatty()

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.

plat/pdpv7/libsys/isatty.c

@@ -1,8 +1,8 @@
 int isatty(int fd)
 {
-    char* p;
+    unsigned u;
 
-    if (gtty(fd, &p) < 0)
+    if (ioctl(fd, /*TIOCGETD*/(('t'<<8)|0), &u) < 0)
         return 0;
     return 1;
 }