ioccom: define ioctl cmd value that can never be valid

Its use is for cases where some filler is needed for cmd, or we need an
indication that there were no cmd supplied, and so on.

Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D29935

newlib/libc/sys/rtems/include/sys/ioccom.h

@@ -84,6 +84,9 @@ typedef unsigned long ioctl_command_t;
 #define	IOCPARM_IVAL(x)	((int)(intptr_t)(void *)*(caddr_t *)(void *)(x))
 #endif
 
+#define	_IOC_INVALID	(_IOC_VOID|_IOC_INOUT)	/* Never valid cmd value,
+						   use as filler */
+
 #endif
 
 __BEGIN_DECLS