mem: support using memfd segments for in-memory mode

mem: support using memfd segments for in-memory mode

Enable using memfd-created segments if supported by the system.

This will allow having real fd's for pages but without hugetlbfs
mounts, which will enable in-memory mode to be used with virtio.

The implementation is mostly piggy-backing on existing real-fd
code, except that we no longer need to unlink any files or track
per-page locks in single-file segments mode, because in-memory
mode does not support secondary processes anyway.

We move some checks from EAL command-line parsing code to memalloc
because it is now possible to use single-file segments mode with
in-memory mode, but only if memfd is supported.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>