Add support for the read-only attribute for cuda::memory::host::register()

[eyalroz]

The cuda::memory::host::register() currently takes 3 flags:

• register_mapped_io_space,
• map_into_device_space,
• make_device_side_accessible_to_all

(the latter has a typo in the implementation name)

... but recently, it seems there is a new, fourth possible flags: CU_MEMHOSTREGISTER_READ_ONLY. We should add support for it, depending on the CUDA version.