cpuid: Now we support extended feature testing.

For now, we'll just have this and later on I'll make it support
writing it also using cpuid_write_info.

Signed-off-by: Ahmed Samy <f.fallen45@gmail.com>
cpuid: fix example compilation

Signed-off-by: Ahmed Samy <f.fallen45@gmail.com>
io: io_always, and zero-length operations support.

A zero-length read should complete immediately, even if the fd isn't readable.
Wire this up, and expose it for callers to use.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>