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 enc...