nitro_enclaves: Add sample for ioctl interface usage

Add a user space sample for the usage of the ioctl interface provided by
the Nitro Enclaves driver.

Changelog

v9 -> v10

* Update commit message to include the changelog before the SoB tag(s).

v8 -> v9

* No changes.

v7 -> v8

* Track NE custom error codes for invalid page size, invalid flags and
  enclave CID.
* Update the heartbeat logic to have a listener fd first, then start the
  enclave and then accept connection to get the heartbeat.
* Update the reference link to the hugetlb documentation.

v6 -> v7

* Track POLLNVAL as poll event in addition to POLLHUP.

v5 -> v6

* Remove "rc" mentioning when printing errno string.
* Remove the ioctl to query API version.
* Include usage info for NUMA-aware hugetlb configuration.
* Update documentation to kernel-doc format.
* Add logic for enclave image loading.

v4 -> v5

* Print enclave vCPU ids when they are created.
* Update logic to map the modified vCPU ioctl call.
* Add check for the path to the enclave image to be less than PATH_MAX.
* Update the ioctl calls error checking logic to match the NE specific
  error codes.

v3 -> v4

* Update usage details to match the updates in v4.
* Update NE ioctl interface usage.

v2 -> v3

* Remove the include directory to use the uapi from the kernel.
* Remove the GPL additional wording as SPDX-License-Identifier is
  already in place.

v1 -> v2

* New in v2.

Reviewed-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Alexandru Vasile <lexnv@amazon.com>
Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
Link: https://lore.kernel.org/r/20200921121732.44291-17-andraprs@amazon.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>