add port io functions to sys/io.h

based on proposal by Isaac Dunham. nonexistance of bits/io.h will
cause inclusion of sys/io.h to produce an error on archs that are not
supposed to have it. this is probably the desired behavior, but the
error message may be a bit unusual.

include/sys/io.h

@@ -4,8 +4,9 @@
 extern "C" {
 #endif
 
-int ioperm(unsigned long, unsigned long, int);
-int iopl(int);
+#include <features.h>
+
+#include <bits/io.h>
 
 #ifdef __cplusplus
 }