Changed cncheckc() interface so that it is 8-bit clean - return -1

instead of 0 if there is no input.

pcvt_drv.c:
Partially fixed pccncheckc().  It returned a boolean value instead of
the character that it fetches from the input fifo (if any).  I think
it still discards characters after the first for multi-char input.

sys/i386/isa/pcvt/pcvt_drv.c

@@ -1204,7 +1204,7 @@ pccncheckc(Dev_t dev)
 	cp = sgetc(1);
 	kbd_polling = 0;
 	splx(x);
-	return (cp != 0);	/* did someone press the "Any" key? */
+	return (cp == NULL ? -1 : *cp);
 }
 #endif /* PCVT_FREEBSD >= 200 */