add ioperm/iopl syscalls

based on patches by orc and Isaac Dunham, with some fixes. sys/io.h
exists and contains prototypes for these functions regardless of
whether the target arch has them; this is a bit unorthodox but I don't
think it will break anything. the function definitions do not exist
unless the appropriate SYS_* syscall number macro is defined, which
should make sure configure scripts looking for these functions don't
find them on other systems.

presently, sys/io.h does not have the inb/outb/etc. port io
macros/functions. I'd be surprised if ioperm/iopl are useful without
them, so they probably need to be added at some point in appropriate
bits/io.h files...

include/sys/io.h

@@ -0,0 +1,13 @@
+#ifndef	_SYS_IO_H
+#define	_SYS_IO_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+int ioperm(unsigned long, unsigned long, int);
+int iopl(int);
+
+#ifdef __cplusplus
+}
+#endif
+#endif