Make Ioctl::opcode a method instead of a constant

[coolreader18]

This lets the Ioctl trait be compatible with ioctls that take a runtime length value, e.g. these evdev ioctls from input.h:

#define EVIOCGNAME(len)		_IOC(_IOC_READ, 'E', 0x06, len)		/* get device name */
#define EVIOCGPHYS(len)		_IOC(_IOC_READ, 'E', 0x07, len)		/* get physical location */
#define EVIOCGUNIQ(len)		_IOC(_IOC_READ, 'E', 0x08, len)		/* get unique identifier */
#define EVIOCGPROP(len)		_IOC(_IOC_READ, 'E', 0x09, len)		/* get device properties */

These generally specify the length of the passed buffer, so that the driver can copy data into userspace (à la read()). rg 'len\)\s*_IO' /usr/include gives 26 results on my system, so they're not rare, and it'd be great to be able to use the Ioctl trait to wrap them.

Prior art: nix::ioctl_read_buf!()

[sunfishcode]

This looks reasonable to me. @notgull do you have any thoughts on Ioctl's OPCODE changing to opcode()?