Add icoctl_readptr and readwrite_ptr macros.

[ghost]

This macro is missing and are used when communicate with some USBFS ioctl commands in Linux. For example:

#define IOCTL_USBFS_IOCTL _IOWR('U', 18, struct usbfs_ioctl)

For details you may look at my experimental WIP usb rust driver at
https://gitlab.com/mike7b4/usbapi-rs where this macro is used...

[Susurrus]

Failures at least partially resolved by #966.

[Susurrus]

It's not clear to me that either of these are actually needed. Could you provide some more information on what problems you ran into? My initial questions are:

• For your ioctl_read_ptr example, why doesn't ioctl_read work?
• For the readwrite ioctl (I'm referencing the Linux source here), why didn't you do ioctl_readwrite!(usbdevfs_ioctl, b'U', 18, usbdevfs_ioctl);? That should be correct.

[ghost]

It's not clear to me that either of these are actually needed. Could you provide some more information on what problems you ran into? My initial questions are:

* For your `ioctl_read_ptr` example, why doesn't `ioctl_read` work?

* For the `readwrite` ioctl (I'm referencing the Linux source [here](https://github.com/torvalds/linux/blob/master/include/uapi/linux/usbdevice_fs.h#L185)), why didn't you do `ioctl_readwrite!(usbdevfs_ioctl, b'U', 18, usbdevfs_ioctl);`? That should be correct.

#define USBDEVFS_CLAIMINTERFACE _IOR('U', 15, unsigned int)
this ioctl expect a pointer as third argument eg:

C:

{
    unsigned int claim = 0;
    ioctl(fd, USBDEVFS_CLAIMINTERFACE _IOR('U', 15, unsigned int), &claim)
}

ioctl_read!(usb_claim_interface, b'U', 15, u32) translated to:

ioctl(fd, USBDEVFS_CLAIMINTERFACE _IOR('U', 15, unsigned int), claim) reference.

same problem for readwrite!...