Revamp Hurd ioctl typing magic to handle `struct' keyword in the
ioctl command macros, so we can use Unix-compatible headers that
make use of the _IO* macros.
* sysdeps/mach/hurd/bits/ioctls.h (_IOC_ENCODE_TYPE,
_IOC_ENCODE_TYPE_1, _IOC_ENCODE_TYPE_2, _IOTBASE_struct): New macros.
(_IOR, _IOW, _IOWR): Use them _IOC_ENCODE_TYPE(t) instead of _IOT_##t.
(_IOT_int, _IOT_char, _IOT_short): Renamed to _IOT__IOTBASE_*.
(numerous ioctl command macros): Restore `struct' keyword.
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